Well, here's a thought...

I think that when that baby is born he should ask her to do a paternity test. No joke. If she refuses, then you know you've got a cheater on your hands. I was not married to my husband when we had my middle son. I knew 100% it was his, but if he demanded a paternity test, I would have taken it instantly. Without question. Of course, middle son is practically his clone, so... (Actually saw a pic of mother-in-law when she was 6 and she had a similar short hair cut as middle son, well, she's like making this angry screaming face in the pic. LOL. If it wasn't for the yellowing, age, and fact that her brother is also in that pic, it looks like my son! No joke. He even makes that exact face when he's mad. )

Anyway, he should get a paternity test. He should. And if she refuses and blinds him, you will know she's cheating. If you have nothing to hide, then there is no reason to refuse.