Cut back to the phone call, I start screaming at him. "how could you even believe this" etc. He responds "how could I not believe it?" At this point, I pretty much went sideways. Can he think so little of me, care so little about me that he would believe this "friend" over me?

Maybe he just has such little confidence in himself that he thinks no one would want to be faithful for him? I don't know the guy so this is just the most favourable spin I can put on his behaviour from this bit of conversation. He sounds very avoid-y: if you have to check his accounts to see what is going on, maybe he's not ready for a long-term committment after all.

Sorry this has happened.