Advice!! Cheating or not

Okay so my boyfriend was taking a shower and left his phone by me. He has a folder full of my nudes and every once in a while I'll take a peak just to see what he keeps and such. Well last night when I looked I found boob pictures of my best friend. She tends to send them to me before she sends them to her bf so it was a picture off my phone. Like my phone was in the pic he had taken it while I was sleeping beside him on our vacation we just got back from. I asked him why he had it and he really didn't have a reasoning. He did however say one of his friends like her (she has a bf..) and he was going to show it to him. I really don't know what to do. He regrets it and was really upset because I almost left him. But honestly advice what to do?