If your lives are completely separate from this lady & her child otherwise, I would just accept it as a part of him.

If he shows love & affection to this child, that is a good sign. His child will grow up from the start knowing that parents don't always live together. The important part is, this child is lucky in that s/he has two parents who care. If he had turned his back on this child, then I would be more concerned about his character...IMHO. It says a lot about the kind of man he is.

Support him in this. He will appreciate you so much more if you do.