Originally Posted by Ambulance Chaser
As a great philosopher once said, "You gotta know when to hold 'em, know when to fold 'em." Standing on principle in this instance is only going to make the GF mad and the TS sexually frustrated.
Apparently he had options before she came along.
Someone he's been seeing for a couple of months isn't worth all this trouble. Explain the situation and if she can't move on...move on.