She need to go home and change.
Wow! Is there a reason? Do you want to put your wild side of the relationship on display in front of your brother and friends?
No, I just don't want her to wear that shabby dress.
Your gf can wear whatever she want. You can't, or shouldn't, control that.
She should dress decently.
Your partner can dress however she likes. You can't control that. Try to come to some compromise.