My Friend's Fiance Just Messaged Me on Grindr

Hey everyone! I've never been on Reddit before, but I knew that I could get some good advice here so here goes:I had a friend in college who we'll call Keith. We were never really that close, but we considered each other friends. He was obviously gay to me, but he was dating a girl at the time so there was never anything between us, nor did I ever really have feelings for him. There was another guy who we'll call John that I hooked up with a few times and was overall on friendly terms with as well. I don't know if they knew each other or not at the time as they didn't start dating till after college.Fast forward about 3 or 4 years and Keith and John are now engaged to be married. Judging by social media they seem like a perfect couple. They both come from very well to do families, and their lifestyles match up really well. They own a house together, have a cute dog, and are always taking pictures together in exotic locations on vacation. In fact, when I first saw they were dating, I thought they were perfect for each other. They've always seemed very happy.Meanwhile, I'm over here perusing Grindr and last night I received a message from a guy with "curious" in the headline. He didn't have hardly any info on his profile except his age, so I didn't respond when he asked what's up. Then, a few hours later, he sends a pic, and it turns out to be John. I'm one of those headless torsos, so apparently he didn't recognize my chest and didn't know it was me. I had no intention of hooking back up with him again, but I wanted to figure out if Keith new about it (maybe their relationship is open?). So then I respond, and we have the normal introductions, and then he asked for a face pic in which I said it depended on what he was looking for. He said he was looking to mess around a little tonight. I asked if he could host because I knew they own a house and live together, and he said no. He overall seemed to be acting like he was curious and still in the closet (he even said he isn't out when I asked despite their relationship being all over social media). He was even wanting to hook up in his car when I said I couldn't host either. I was also concerned that maybe someone was posing as John (do people actually do that???), but he also sent a dick pick (unsolicited), and I recognized his dick and his living room from FB pictures. Finally, I told him I didn't want to hook up in a car, and that I was too tired and he blocked me. To me, that sounds very much like Keith doesn't know it's happening.I took screenshots of the entire conversation as evidence.I haven't talked to Keith in about 3 years since graduating college, but I'm wondering, do I have an obligation to tell him? Or should I take the "ignorance is bliss" approach and let Keith marry someone who may be cheating on him. I don't want to destroy a relationship or get involved at all really, but I feel like I have to tell him.If I do have an obligation to tell Keith, how should I go about it? I don't want to tell him in public because I don't know how he'll react, and I don't want to put him in an embarrassing position. I was thinking to have him over to my house to tell him, but I don't know how to approach it. As I said, we were never that close, so I'm not sure how I would initiate the discussion with him after all this time.I don't know if Keith knows anything about John and I hooking up back in college, but I'm also worried about Keith's reaction to telling him about John if he knows we hooked up. Maybe he could construe that I'm trying to break them up for my benefit?I'm not really sure how to proceed, but I had trouble sleeping last night and have been thinking about it all day, and I thought I'd turn to the Reddit community for help. Any advice you guys have for me on how to handle this situation is much appreciated.Thank you, guys! WayfarerOK