I mean, she had told him she was with the friend at the same store, but of course being who he is and having such low self-steem it's natural to feel like crying when seeing that, even if you know rationally that it isn't like that.
Best case scenario, he's crying because all of the sudden ugly feeling he got when seeing them together and feeling sad about not being able to trust in her as much as he really wants to. Would be a nice spin on the trope it seems to be going for.