I would tell her.
She's pregnant, and tbh, I think that's about the last time you can warn someone what they're tying themselves to for the rest of their lives.
She has options here that she can't make when the baby is born.
He is already showing a startling lack of commitment to her and their child by continuing to have sex with other people. Sorry, did I read that the two of you use condoms most of the time? ... I just think that if he's not even using them 100% of the time, and he knows you're having other sexual partners, that's horrific on his part putting his partner and baby at risk like that.
I'd been FWB with someone, it was all knocked on the head cos he was a bit of a dick.
I heard from him one day, I knew he had a girlfriend, he told me she was pregnant, from Facebook (me and him were friends on there) she seemed so lovely. I congratulated him, he followed up by telling me that he was happy but couldn't help but treat himself to a new girl to shag a few times a month. He saw it as his hobby. I knew that he was having unprotected sex with his partners because he was very reluctant to use condoms when he was previously had sex. I really regret not forwarding her those messages.
He came up on suggested people about a year ago (I deleted him pretty sharpish after that conversation as I thought that it didn't feel like my place to blow up this poor woman's world and didn't want to see photos of him acting like he was lovely when he was doing what he was)
They've now got multiple kids, and theyre married but she looks absolutely miserable.. I wonder if she knows, or maybe has so many clues that he's snagging away, but just can't prove it.
I think given the opportunity again, I'd let her know.
We all know the gaslighting, and shit that goes along with cheating partners.