It could be that she has been using a new nursing position or nursing without support pillows. Or baby may have bit her, or clamped down hard?
My sister just went through cracked bleeding nipples and they found her baby was tongue-tied. She got Newman's triple nipple cream and a better nursing pillow/nursing stool, and things improved dramatically. She did get thrush a bit later, but genetian violet killed it fast.
You could suggest she see a l/c and get a nipple shield, or pump the one side until it heals. She should be able to get her baby back on both breasts.