Worried for my friend...

Long story short here are the facts:
He's "technically" single.
He's being used as side action by two different girls. One supposedly being a fiance to another man, while the other is in a bad relationship.

We talked and he knows it isn't the best situation. My question is there ANYTHING I can do is just let nature take it's course and just wait till they either get caught or sick of this?

I just don't get it. He has been cheated on by someone he was engaged too as well. I just- I can't understand his mindset at the moment.

Any advice? Will I just have to let life happen to him or can I save him somehow?

Two steady lovers no strings attached for your friend and you're worried...lol

I wouldn't be so worried, most 20something males I know would kill to be in his position.