I've had mastitis 3 times but it has always been associated with breastfeeding. But I know that it can occur at any age if the skin of the nipple is cracked, broken or scratched. but it can also be from other lymph nodes swelling and draining to the lymph in the breast.
Have her apply cold cabbage leaves to the affected breast several times a day it reduces swelling and helps promote the draining of infections. Also have her take acidopholous capsules to regain the proper balance of bacteria in her body. She can also take Grapefruit Seed Extreact (GSE) internally - it is a naturally occuring antibiotic, anti-parasitic, and anti-fungal. She can also apply it topically (but it will itch really terribly so you must dilute it well 2 drops in a 1/4c water is good for a topical wash) White potatoes shredded and applied to the breast also helps. Lay down and cover the breast in the potato and let it sit over night or for at least 30min.
Do not wear a bra while the infection is there! Do not wear tight clothing. And take many hot!!!! showers/baths! And it hurts but try to rub the breast in a circular downward motion to help release the infection.