That's a real corner case, and in any event, that kind of piss-poor approach to open relationships doesn't deserve to be respected, especially if they're too immature and stupid to deal with someone inevitably bringing it up with the other partner.
Certainly not to the point of assuming such is the case without some actual reason to do so.
The crimes of others are not the responsibility of others in such cases.
Now if one actually had reason to believe he'd kill her, that obviously changes matters, but as something that's purely in the realm of conjecture, that's grasping at ephemeral straws.
You're getting into paranoid territory at this point.
They're obviously not stupidly happy together if she's cheating on him, and getting out sooner rather than later would ultimately be better for him, especially if there aren't any children yet.
Then one did one's due diligence and since it's out of one's concern or care, it's irrelevant if he chooses to believe her.
I'm sorry your parent went through that, but that's too much of a corner case for me to believe it's something that anyone should really give weight to.