Why make up these fantasies about someone we don't know? Wolfgang's scenario just doesn't give enough information to make those kind of judgements.

And in any case, when did being "a liar and a cheat" stop a mother being a good mother in the things that actually matter to her child?

If being honest to "both prospective fathers" would have been to the disadvantage of her child, it would have been the wrong thing to do.   The interests of a child outweigh anything else.