Unknowingly helped a guy cheat

Several weeks ago, I matched with this really cute twink on Tinder. He was just my type. At first, the conversation(s) seemed to go nowhere but eventually he asked to hookup. I obliged-he came over and we fucked.A few days later, I was out with some friends and saw an old friend from high school. We've hardly spoke in the 5 years since graduation. Lo and behold, she introduced me to her boyfriend-the guy I hooked up with. (I was living in a small town at the time). I asked how long they had been dating-she said 6 months.Which means, unless they are in an open relationship, I helped this guy cheat on my friend from high school.I have a feeling of extreme guilt, because if I had known, I never would have hooked up with him. Nowhere on his tinder profile did it mention a girlfriend. And I haven't hooked up since because I still feel like shit about the whole thing.So should I tell my friend? Or just continue and try to get over my guilt?