Woman here. I completely disagree. If this girl is wearing shorts that make her ass hang out at her workplace (assuming her workplace is not a place where booty shorts are okay), then she absolutely is not dressing appropriately.

It's one thing for a boyfriend to say "I don't like you wearing that" which is completely within his rights. It's another for him to say "don't wear that."

If this girl is being constantly hit on and has her ass cheeks hanging out at work, I don't see anything wrong with her boyfriend bringing it up to her that it makes him uncomfortable. They've been together for 5 years--you should be okay to say "hey, this makes me uncomfortable" in a relationship.

Does he have the right to force her to stop wearing them? Nope. But he sure can, and should, voice his concerns, especially if she's constantly getting hit on. Put it this way: I've been with my SO almost 3 years--if he did something which resulted in him constantly being hit on by women, I'd be uncomfortable with it too.