Generally speaking the use of herbals for healing or ritual work may often be included as a practice by those that self-identify as Pagans including also practitioners of the Religion of Wicca. That doesn't make use of herbals mandatory or anything that is intrinsically Pagan or a part of the actual practice of the Religion of Wicca. The use of herbals has and is found to some degree within every culture and amongst many belief systems. So think of herbals rather as a universal skill-set amongst humans that a Pagan or a Wiccan might personally choose to learn and use today. What a Pagan or Wiccan might access about herbs depends on his or her training, availability and how this information is packaged/targeted. Herbal correspondence systems (detailing herbs and their purposes) are created, borrowed, adapted, cobbled together and so on based on the needs and beliefs of various cultures, familiarity and availability. For example, one finds lavender popularised in modern generic Pagan Witchcraft as one of the 'Thirteen Herbs' that Witches are supposed to have on hand to use in talismans and charms. Not surprising it is listed given its wide spread popularity throughout history and notably in late medieval sources that such correspondence lists often draw upon. The attributed virtue(s) of lavender depend on what sources a Pagan Witch considers to be valid. I've seen everything from enchantment and happiness to chastity, purification, love, protection, sleep, making wishes...etc. As a Witch, I believe anyone using a herbal needs to test a correspondence and see if it works. If it doesn't, find something else that does. If it does work, then keep it. PLEASE NOTE however that Lavender oil taken INTERNALLY in too large a dose is narcotic and can cause death. Your friend is describing an external use and if her experience(s) have been that the oil is useful externally as described, then that would be a practical correspondence.
As for information on Lavender, the Labiatea family is notably Lavendula officinale or vera (English Lavender) spica and latifolia (Spike Lavender) and stoechas (French Lavender). It is indigenous to the western Mediterranean and today cultivated in many places in the world. In addition to being sold fresh or dried, lavender oil mostly from English Lavender has long been considered valuable for trade mainly for perfumery but also for medicinal uses. Although it isn't mentioned in England until the late 16th century, lavender grown in England is considered to be the finest quality. Essential oil is produced only from the flowers and flower stalks. It and Lavender Cotton (an unrelated plant) was taken with those migrating to North America and used to perfume clothing mainly though it often didn't do well in northerly climates. Stoechas or French Lavender from southern France was reclassified as L. delphinensis, the Lavender of Dauphine, and L. fragrans. It is smaller and darker, smells more like rosemary and is found usually near the coastal areas and on islands like the Hyeres (Romans called these isles Stoechades). It is probable this was the main source of Roman lavender. White Lavender found at extremely high altitudes is considered to be a form of L. delphinensis (albino lavender). It was considered rare and listed in various accounts of royal medieval gardens. Spike Lavender sometimes referred to as Lesser Lavender or Nardus Italica is found in mountainous areas of France and Spain but it is harder to cultivate in open fields. Still its flowers have three times the amount of essential oil (Spike Oil) though less fragrant and thus considered to be second-rate. This is usually the lavender that is associated with ancient cultures such as Greece (Nardus) or Rome (Asarum) or Spikenard (in the Bible).
Medicinally, lavender was used until mid 18th century although as with any herbal mentioned in any historical capacity, such references should not be taken as automatically safe. Some of the uses historically and today are listed below and as you can see, it's usefulness really depends on what you believe lavender can do.
EXTERNAL uses of Lavender Oil
In France and Spain, lavender oil is used for dressing wounds and other places record the practice of making it into ointments and baths, notably to help with colds.
A few drops of oil in a hot bath or footbath were said to relieve fatigue.
It is used for swabbing wounds, treating sores, ulcers and burns. Essence of Lavender is still a common household remedy in parts of France for bruises, aches and pains and bites.
In the 18th century again it was held to be good for serpent venom, dog-bites or anything venomous applied to the wound in a poultice.
Rubbing Lavender oil externally is said to be good for stimulating paralysed limbs. When mixed with spirit of wine, it was known as Oleum Spicae, famous for stiff joints.
Lavender oil is used to kill lice and parasites by veterinarians
Also used to embalm corpses.
Depression, delusions, visions, a few drops of lavender oil were rubbed over the temple.
(some) EXTERNAL uses of Lavender
Dried lavender is used with linens to protect against moths and keeping away flies and mosquitoes who don't like the smell.
Dried or fresh lavender was used in baths and to perfume churches and houses during festivals or burnt in bonfires to ward off evil spirits.
INTERNAL uses of Lavender
You can still buy Lavender tea that is prescribed for fatigue and headaches or Lavender water that is applied to the temples for the same reasons.
Lavender oil or a spirit made from this was considered to be an excellent restorative and tonic (again caution here with any internal use being described) for spasms, colic, nervousness and faintness. External attributes include relieving hysteria, palsy, cowardice, rheumatism, neuralgia, toothache and sprained muscles.
It was sometimes taken with milk and sugar to dispel flatulence.
Lavender was used in medieval times and sometimes today as a condiment like a marmalade or conserve and flavouring dishes with it was held to be good for one's stomach.
Candies and lozenges were regarded as mildly stimulating and helping one's appetite.
Lavender-scented honey is much prized from hives near to lavender plantations and fields.
You can still find Lavender drops used as flavouring or to colour being also used for faintness.
One can also find Red Lavender a cordial consisting of lavender oil, rosemary, red sandle wood, nutmeg and cinnamon bark that is sold for indigestion.
Palsy drops or red hartshorn has been sold for several hundred years in England and contains nearly thirty ingredients including fresh lavender flowers. This is supposedly good for everything from epilepsy to 'distempers of the womb' convulsions, nerves, vertigo, memory loss and infertility. It was taken in wine or juice or in milk or water with sugar.
Distilled water from lavender is used to gargle and guards against loss of voice.
Culpepper (a medieval English physician who wrote a popular herbal manual) listed 'a decoction made with the flowers of Lavender, Horehound, Fennel and Asparagus root, and a little Cinnamon, is very profitably used to help the falling-sickness (epilepsy) and the giddiness or turning of the brain.'
Lavender is listed as one of the four ingredients of "Four Thieves Vinegar', a concoction supposedly created by thieves during the Black Plague to protect themselves and by which they 'sold' to mitigate their sentencing. There are several 'versions' of that recipe around last time I checked.
Probably more information than you needed. Again, if your friend has found it useful as you described, then this would be a practical correspondence. Whatever attributes she is ascribing to it however, you did not specify but as you can see, it depends on what you believe lavender will achieve.
Hope something in all that was helpful.