When I was 17 yo, I had my 1st LTR. For context, at that time she was HB8. This was before RP, so my behavior was beta. She broke up with me after 2 months.

I wanted her back and we got back together after 2 weeks or so. In this time, my best friend started talking to her. I found out they were together, after a long time, in the same time I thought I have a LTR with her. They've also kept behaving normally, as nothing happened.
There was another guy that was friends with me and my best friend and he knew about it and kept this from me. He had to pick a side and he picked my best friend instead of me. We were meeting almost daily and he still didn't tell me.

I didn't speak with these two guys for 1 year after that, but then we started talking again, but never like before.

Now the other guy wants to start a business and because he has no one to trust, he wants me to partner with him, because he knows I keep my word and I have work ethic.

I don't trust him, bad partnerships are really daunting mentally and financially. On the other hand, the idea is really good, it has potential. If this blows up, I could achieve financial freedom.

Should I forget that he kept that from me (taken into account we weren't even 18 at that time) or people never change?