Found Tinder on my bfs phone????

So over the past few weeks I have noticed my bf of 3 years acting a bit different, he changed the password to his phone randomly (we have always let each other have passwords to phones in case of emergencies), when I asked him he told me that his friends at work found it out and were logging into his phone and messing with him. Fair enough, but after he told me it he changed it again and once again he said the same story and reluctantly told me the new one. He went in the shower tonight and I know it’s wrong but I felt suspicious about the whole thing, so I went in his phone. And found the Tinder app. He had a recent profile set up but no messages with anyone, when he finally came out of the shower I broke down to him and asked why on earth he had it. He told me that he had no idea it was on there and that his work friends must have set it up to “mess with him” and told me that he loved me and would never cheat on me. I really want to believe him especially because there are no messages but I can’t help but feel like I can’t. What would you do in my situation?

Update: heard you all loud and clear, pretty sure I already knew the truth but needed to hear it, left his ass.