Ask about it! I own loads of mens' clothing because it's more comfortable than womens'. Like I don't like sleeping without underwear but don't want fabric flossed up my ass all night so I'll wear boxers, stuff like that. She might have similar reasons.

Past that point I actually think it's up to you to decide what you are or are not comfortable with since there's no hard moral answer to it. I don't like when guys I'm with keep their ex's stuff (probably because they can't wear it but idk) but my boyfriend happily wears my ex's clothes. You can draw your boundaries where you need to but remember it's also her right to say no, and from there you can discuss that.