I think before you say anything, get your facts 100% (or as close to as you possibly can) straight. Talk to your friend and find out exactly what she saw. She may also be able to provide you with some support.
As far as telling your bf goes, my only advice is to tell him exactly what you just told us. Unfortunately I dont see any way of sugar coating it but it does seem like you are genuinely remorseful. Honesty is always the best policy, and im sure you don't want this secret hanging over your head for the rest of your lives as a couple. Hopefully it is something that you guys can worth through together.
All the best
RQ