Wow, what a douche move from a so-called friend. Yes, you should absolutely tell your boyfriend. He needs to know that this guy is 0% friend and 100% asshat. Real friends don’t try to pork a buddy’s girlfriend.
The length of time they’ve been friends is irrelevant, as is the length of time you’ve been dating. Even if they’d been pals for 50 years and you two had only been on one date, wrong is wrong, and your guy needs to know the truth. I guess there’s always an outside chance he’ll believe the person he’s known the longest, but hopefully he’s smarter than that. Why would you make up such a story?
I realize you might feel guilty about breaking up their friendship, but you aren’t. The friend did that when he chose to hit on you. I also realize it would be easier just to let it slide — no one likes drama — but that would be putting your need to avoid conflict ahead of your boyfriend’s need to know the truth. He trusts a guy who is making a fool of him. You can’t let that continue another day. Who knows what other twatty things this guy has done behind your boyfriend’s back? A dude who will make a pass at a buddy’s girlfriend is capable of almost anything.
This one’s a no-brainer. You have to tell him so he can unload this piece of shit and flush it out of his life. He won’t like it, but telling him is the right thing to do. Unfortunately for all of us, the right thing is usually the hardest thing.