Added thought... and also an answer to your initial question whether anyone has experienced this, yes.

When I was a lot younger and met my first wife to be (mistake, I'll say ahead of time) she came onto me and I got her phone number etc. The first time we talked she admitted she was with someone, had a bf etc but implied she wanted to go out with me. I simply told her - when you're single and not involved with someone, I'll consider it.

Days later she tells me she broke up with her bf and we started dating at that time.

Unfortunately her willingness to do so initially was a red flag that I was too naïve to pay attention to and years later - after married for 6 yrs and many instances of suspicions she was called out by our child of four of being with other men. (daughter didn't know the difference just told daddy about her stays with other men...while our daughter was present!)

Likely his pursuit and interest in other women while still involved is a serious red flag anyway so I stand by my original comment to cut off communication and walk away.