It stikes me that if you don't feel guilty, you can't be that much in love with your boyfriend, and maybe just with him out of habit.

I judge everything by how I would feel if it happened to me.
How would you feel if your boyfriend kissed another girl?
If you wouldn't be that bothered, you shouldn't be with him.

If you do love your boyfriend dearly be very careful with the truth, as people that tell it just to clear their own conscience are being selfish. If it is never, ever going to happen again, him knowing won't make his life any better. If you think there is a reason this happened and it needs discussing, you need to be honest with him.
And be honest with the friend too, tell him you just want to be frinds. Although, I'd be amazed if you could be friends after all this, not for a while anyway.