Ahw, this is one kind of dilemma i don't ever want to get into. 8 years is not a short span of period. The child could get confused by then and I believe would be hesitant to go with the mother. If I were your friend, I'd tell the mother not to get the child instantly, but maybe eventually I guess? Like for instance, we could go on a picnic on a weekly basis so as to let the child know her better. Then from there, little by little, she can be with the child alone for longer periods of time. She should first develop to have the child's trust and love.
She could not just get the child and tell her, "I am your real mother".