Please help! Its long, im sorry.

So last night, my partner went out. No biggie. He's allowed to do as he likes. I stayed home and had a Chinese with my friend so had a lovely evening also. 1:30am he calls to say he's on his way home. (We are 15 minute walk from town, where he was). 2:30am he wasn't home. He eventually calls and said that he's sorry he was in a taxi when our friend called him (she's been close with him for 5 or so years. Maybe longer. I've known her 13 years as we used to hang out in a big group but lost touch. Only started speaking again 2 years ago when me and my partner got together, I have known my partner since school, he fancied me like crazy but I was never interested). Anyway on this phone call he proceeded to tell me that our friend had cheated on her partner last week. My partner said he couldn't back her up because if she's done something wrong then she needs to fix it. She then proceeded to say "oh what like you did with your girlfriend" he questioned her and she ended up saying that he kissed a girl at a festival while I was in the UK taking my granny to her first chemo appointment. (This was around August time). Now we obviously had an argument. I cried. He got upset. Said he didn't. He wouldn't. Deep down I believe him. Hes has a big thing on cheating. But.... I said can you tell me 100% you didn't do it. He said he can't because he was absoloutley wasted by 7pm waiting to go home and doesn't even remember the taxi journey back into town. This doesn't make it okay but he doesn't know if it happened. In my heart I don't think he did. But there's a chance he did because he doesn't know. I've told him that he needs to message our friend today when they are both sober and find out what happened. Now again, for some context, my partner is amazing. I've never ever felt like "this is the one" before. He's my best friend. He's taken on my son (who I adopted) so easily and loves us both. I couldn't ask for a better partner to do this life thing with.... however, I also know he and our friend had a thing waaaaay back before we were together, never went further than kissing on a night out. I trust them both, they hang out, they're also close friends and I have no issue with it. Could she have said what she said out of truth or possibly lied out of jealousy? I don't want to think my boyfriend cheated on me but I also dont want to think my friend has hidden it from me. So either way, I feel like by the end of the day I'm either loosing two friends (apparently another of our female friends know) or loosing my boyfriend. What would you do? He's never "done" this before. Weve never been in this situation. If its true, do I let everything go purely because he got himself in such a state that our relationship didn't cross his mind in that moment? Do I work on it? Please help me. I love this man with my whole heart. This is the guy I want to marry (mutual). We made the decision to come off the pill a good few months ago to see what happens and I don't want to throw our lives away. We've also just moved into a new flat together and have a wonderful life. I know this is long but im just looking for some opinions! 🤍