。
One of my friends took care of her.
Literal
Friend-of one-person [subject-が] her-of care [object-を] see-[past-た].
面倒を見る ('to take care of, look after') is a core idiom — literally 'to look at the trouble/bother,' i.e., to handle the trouble of caring for someone. 友達の一人 ('one of [my] friends') uses the の-linked 'one of' pattern. Common caregiving register.