Her husband dresses like a slob. What can she do?

“My husband will wear things like ripped jeans, t-shirts with stains and gym shorts when we’re out shopping, at dinner and other stuff in public.
I’ve told him that I’m embarrassed, but he says that I shouldn’t care and that he’s not trying to impress anyone, so ‘why not be comfortable?’
How can I get him to dress presentable in public?”