As others have said, if the therapist did say that to your friend’s child, it was wholly wrong and inappropriate.
Having said that, what clearly matters most is the feelings of the child in this situation. Obviously, whilst this will have caused considerable distress and anger to your friend, instead of putting energy into fighting with the therapist, taking legal action, publicly tarnishing their reputation, and so on, I’d just focus on the child, which is really all that matters. Presumably the child has already experienced enough turbulence with the separation of his or her parents, and it sounds like protracting that beyond what is necessary wouldn’t be in the best interests of the child.
If your friend can reassure and talk to her child about what was said and make sure he or she isn’t distressed, and if your friend continues to be a great mother as you have said she is, the child will know what sort of mother your friend is, regardless of what anyone else says.