should i tell this to my friend 18+

my friend broke up with his girlfriend in 2024, before the end of our last year in highschool, before summer. after more than 1 year in august of 2025, her ex who i was friends with, reached out to me to ask information about the entrance exam for university, and I was friendly in a way i shouldn't have been (not flirty or weird), but cracking jokes and being my usual funny, which is how it always goes with me, but this time in a way that you wished the other to think that you were funny, since the other was an attractive girl. I don't think I meant anything more than that, but deep down I was actually attracted to her and harbored a closeted interest, in the small way of simply an attractive girl texting you and you enjoying it. She also asked me to hang out with our ex highschool group of friends and another male friend of mine to grab a few drinks with our highschool teacher who was gonna get married in a few days. I'm not going to lie, a part of me cultivated some kind of vague and loose sexual fantasy of her, all in my head, (im going to say something very cringe), like kissing her or something happening in that hangout. I was also very depressed and cynical in that period and OCD was consuming me, so maybe I didn't really think through things, maybe i thought since they were broken up before summer of 2024 and more than an year had passed, that it technically wasn't wrong, maybe just a bit iffy, but not ethically wrong. And since I wasn't much friends with him anymore I guess it didn't cross my mind that it would not be okay, but recently I became close friends with him again so now I feel regret thinking about it. In the end I know I would have never pursuit a relationship with her or something like that, i know if there was the occasion i probably would have chosen not to kiss her (but in my fantasy inside my head, when i was h*rny I'd let loose and not care abt consequences). But I didn't have time to process that possible character development and chain of rational thinking because that evening when we went grab drinks she mentioned she was seeing a guy and all my interest had faded, nothing happened. So it's not like I realised at that time that what I was feeling at the time was wrong, simply that temporary fling inside my head vanished overnight. I never thought much about it before since I didn't think I did anything wrong (because a part of a few friendly exchanges nothing more happened and i didnt pursuit anything more). But I just realised this year that it was no good, it hit me. I never flirted with her, just a friendly chat that lasted for the lenght of the conversation reserved to exchange info about the entrance exam one day, and also another day about grabbing drinks, but nothing more. If someone were to read my convo with her nobody would think abt it more than as it being just a chat between friends. But I did look at her instagram stories with lust. I did once answer to an instagram story of her saying that eating apricots and a Monster together was criminal. Since that realisation I started ignoring her presence on social completely, i once used to like her stories but as a friend, now i don't do that. I feel like a terrible friend. Should I tell him? It would ruin my friendship with him.