He may not know we're together, so I would give him this benefit of the doubt, but make it clear to him that she is taken.

Let it go unless it starts to bother her. Then I step in and tell him that she is mine. That has worked well for us over the years.

If with the woman I loved more than life, to warn him about the corrosive ironies she used to stop with no ambiguities all flirting except mine.
Except few and rarely needed times when with a gesture she asked me to look at him the way she knew would sent him appologizing while waking backwards as fast as he could.

Sit back and watch

Buy him a beer, shake his hand, take him back to my place for "more drinks", then MURDER.😠🔪

Stab her in the clit

Let him know she is taken, if he continues then the knife comes out 😊