I had this happen on my "contact fingernails" the nails I use to remove polish , apply acetone soaked cotton for gel removal, and the nails I use to swipe a side of nail if gel polish is touching skin. I believe it was contact dermatitis. Mine also tested negative for fungus. I found that it was almost like a callus of dry skin building up under my free edge. I started using a cream that contained A.H.A AND UREA. The combination of the two ingredients softened the skin and returned my hyponychium to normal. Advise her to not do anything that pulls the nail up or separates the nail from bed, like cleaning under nail edge to aggressively or picking the dry skin. Keep area super moisturized oil lotion even antibiotic ointment or hydrocortisone ointment. It should improve if she is diligent. Gel polish may be to hard and cause the nail plate to shift around also causing trauma to hyponychium. I would only use long wear polish like vinylux or opi infinite. Limit exposure to removers or add oil to polish remover to help prevent dehydration.