What purpose would be served by telling him? You behaved in an exemplary manner.

If you tell your bf you'll start a huge bunch of problems. You'll never hear the end of it. Every time for the next 100 years that you have a spat, he'll refer to this incident and say something like, "Maybe you would have been happier with Jack. Looks like you missed your golden opportunity." <snark> First, you'll make mortal enemies of your bf and the other guy. At worst, your bf could walk out the door and leave *you*. No, you didn't do anything wrong, but when it comes to sexual moves and propositions, things get very confusing. It's best to keep your mouth shut. If you're desperate to talk about it, talk to a trusted gf or your mother, not your bf. You might think you're just being honest and open, but it's really an admission that you're too immature to handle such mundane matters without help. I'll assume that you're still a teenager and you're not used to sexual invitations from guys. Unless you run away and join a convent, you'll get them all your life. It's just nature. Relax and shrug it off. That's what the other guy expects anyway--he's thinking, if he asks 20 women he might get lucky with one.

I'd say it depends on how well you think the sexual tension between you and your "Friend" is resolved. On the one hand, if it seems to be over and done with and unlikely to present a problem in the future, then you don't exactly have an obligation to tell your boyfriend. Granted, I can see how your boyfriend could have a problem with that if/when he eventually finds out, and if you think he's the kind of person who would, then it becomes something of a judgment call for you as to whether it might be better for your relationship in the long run to get it out of the way now. On the other hand, though... if you notice that your "Friend" is having a hard time dropping the issue, then I would actually recommend telling your boyfriend before things escalate too much further. After all, if you don't tell him and it escalates, you could end up in a situation where your "Friend" starts coming on strongly enough for it to look like an affair to the untrained eye, and where your boyfriend, on account of never being told about the unwelcome advances in the first place, doesn't know enough to see that it's not one. Ultimately it's your call, but those could be some things to consider when making it.