Its not your place to tell her. I've been in your position, I've been cheated on and I've done the cheating. You are not doing this to help her. You are doing this to get back at him and now you want to rat him out because he doesn't want to leave her to be with you.
If you really want to help, stop being the side ho and go find your own man, if you can. If a man is gonna cheat, then he's gonna. You should have enough self respect to not let it be you who he is cheating with. So imo, yes I would want to know. However, there is a nothing a side ho who is ****ing my man knowing he has a girlfriend, could say to me that I would receive or be appreciative about. You would to need to better keep your distance. I'm just saying.
You not even fooling me. I know exactly your motives. You need to just stfu, let him go, get over it, stop being a side ho (if you must be the side ho, play your part, stay in your lane and again stfu) and move on with your life.
You're welcome.