So, one of my best friends dresses the exact same whenever we go out anywhere: very short crop top and short skirt with trainers. I really don't mean to be snobbish bc I have never had a problem with how ANY of my friends dress it's their choice and I love my friends unconditionally, however, SOME of my close friends are very different to me (different social circles and class). She's from a lovely family, and her dress sense is her choice and although I wouldn't dress that way she always looks pretty and I'm always supportive.
The problem though, is that whenever we go out to nice restaurants in London (including the savoy, shard and the ritz, all presents for her previous birthdays) she still dresses in either a short skirt and crop top with trainers or replaces the skirt with jeans when informed that's there is a dress code. I've also told her before that she can borrow anything from me, she only ever borrows my going out tops :/
My family friend has recently opened a lovely restaurant and obviously I have invited her to the opening, this time however I do feel I need to insist on a black tie dress code. I have said it to all 18 of my best friends so she doesn't feel attacked at all. Despite this, she sent a picture of a very short dress with a dropped back and trainers as her outfit. The worst part is she has already paid for the outfit with her own hard earned money and loves it. If it was any other "more privileged" friend I wouldn't mind being quite frank and I also know they're not sensitive and they would understand.
I don't know what to do as I don't want my family friend to think I didn't ask my friends to dress appropriately, especially since he's also my dad's business associate. On the other hand, I also don't want to hurt my best friend's feelings.