I will gladly be tiresome to you and remind you that this does not apply to normalizing a sin.
Who is calling for the normalization of sin? It is a private party ... they should’ve given her the (non-cross) dress code and different-gendered date policy for the private party, and invited her along with everyone else.
If she chooses not to attend due to the policy, all is well ... you did your part, she made her choice.
SnakeDoc