Dear Vengeful:

I'd really need more details on the situation to have any idea!  At this level of generality, the main thought that's coming to my mind is to make it look like he betrayed her to you.  Other women will compete for him less, after that, but not in a way that reduces their respect for you.  He'll know the truth, obviously, but his alternative options to you will be much reduced.  If he's not flattered by the romance of the effort you expended to arrange to trap him so, then yes write him off.

Dear Vengeful:

I'd really need more details on the situation to have any idea!  At this level of generality, the main thought that's coming to my mind is to make it look like he betrayed her for you.  Other women will compete for him less, after that, but not in a way that reduces their respect for you.  He'll know the truth, obviously, but his alternative options to you will be much reduced.  If he's not flattered by the romance of the effort you expended to arrange to trap him so, then yes write him off.

Dear Vengeful:

I'd really need more details on the situation to have any idea!  At this level of generality, the main thought that's coming to my mind is to make it look like he betrayed her for you.  Other women will compete for him less, after that, but not in a way that reduces their respect for you.  He'll know the truth, obviously, but his alternative options to you will be much reduced.  If he's not flattered by the romance of the effort you expended to arrange to trap him, then yes write him off.