So what would you do Stella?
She is quite possibly the most naive girl I have ever known.
She was VERY sheltered in her up bringing and he is her first real boyfriend, she's over 30.
It's a tough situation since the guy is such a close friend.
What do you think about having a conversation w/her regarding people cheating on each other and then asking her "Hey Martha, if Fred was dating someone else as well as you, would you still date him?"
If she says yes, then you don't need to do anything. If she says no way then maybe you could just give her a look of great concern. I don't know!!