I'm literally in the exact same situation as you, but it was not a 10 year separation. I haven't said anything to my "old love", because I feel like his friend knows better than to say anything. If it would make you feel better, you could talk to his friend and tell him that you have feelings for this guy again, and that it would only hurt him if he ever mentioned that you slept with him. Honestly, lying is never the answer, but in some cases, it can be a better decision. Good luck, let me know how it goes!