Wow, is he aware of any of this? I can see where this can get a little uncomfortable and awkward. Is he seeing a therapist?

I think, first off, we must stay honest and not claim to be a "new mom" because that can backfire on you. These are real insiders that feel and probably experienced trauma themselves. I'd explain to them that you are a best friend or wife of the body (?) and wish them the best and get out of that mother roll so nobody gets hurt. I know you didn't know what to do, but I think honesty is the best route to follow.

Have you a way to record what's been happening? This can be useful real soon like for proof or something to present to a T.

I hope things get worked out for you and him.