Most Helpful Girl
I think as long as he's proven himself trustworthy so far, and he didn't text her anything like "I want to kiss you again so bad," it seems pretty harmless. That being said, if you're concerned about it, you should bring it up with him. Make it clear that seeing the text was an innocent mistake, rather than malicious snooping, but that you're not entirely comfortable with the idea of them meeting up or the fact that he didn't mention it. If he's a good guy, he'll find a way to reassure you - which could include not going if it will upset you. If not, he may get very defensive and upset about it. But if you're uncomfortable, I wouldn't sacrifice your feelings for the sake of avoiding conflict.