is it wrong for wanting to check my friends phone?

Long story short. My friend added my ex on Snapchat. I told him multiple times I find it very disrespectful, and I didn't like it that he does that kind of shit. He said that I had to shut up and cry about it. My gut tells me that I don't trust him, and I want to check his phone to see the messages, etc. I want to check it to make the last confrontation more clear. if i need to confront him about just texting her or also about the messages. Also, should I do it before the vacation we have in 2 weeks or before? If I do it before, I'll have hate towards him, but if I do it before, I could ruin the vacation (we are going with more friends).