Conflicted, mental health relationship help

My boyfriend told me he cheated on me at the start of our relationship with an old friend at the time. I believed him but did wonder if it was true. He then told me a lot of other things that happened that he remembers but some of the things he says is not true. He told me me, him, and 2 friends had a convo about me cheating and there were tears and all but this is 1000% not true. Some things he genuinely remembers and can describe in detail is not true. Me and a friend think there is more to it and has something to do with his mental health as he is positive he remembers all these things. Now I am trying to go through everything to see what is true or not in order to help him figure out what is reality and what are his real memories. Do I ask this girl if that really happened and he did cheat or not, it was a long time ago and i don't want to bring up old shit or cause undue stress but I feel I need to know the truth in order to help him. She has no idea this has been said (I'm not even sure if it is real or not). This is causing lots of stress and strain on our relationship now. What should I do, any advice please?