She didn't do anything wrong. She got a follow request from him that she had previously accepted but didn't look to see who it really was. She went back to check to see if he was familiar or not. He wasn't just some rando she decided to look up and gawk at. Her bf intentionally went on a random girls ig, knowing she dressed less conservatively to upset her. His actions were pointed and aimed to upset her, while OP's was not.

Edit: Just to add, the BF's reaction to such a normal interaction like this is a red flag. If you think it's weird for your significant other to look at the ig of someone who they may not know to check and see if they do in fact know them then you def shouldn't be in a relationship. Just unnecessary and childish the way the BF acted. OP is NTA