It's never too late. PLEASE, TELL HIM.
And sometimes it seems that when OW are continually obsessed, it's not because they are delusional, it's because they are still in contact with the wayward.
If she's not letting him go, maybe he's not given her a reason to.
There are millions of ways they could go underground. My ex fucker used the chat feature on a fucking scrabble game on his phone to go underground. Waywards can be some sneaky fuckers.
I would tell the boyfriend ASAP. I think it gives your relationship a better chance as well..