G.♣.
Let her pick out her own underwear. Unless she asking for underwear that costs $20 a pair or something really expensive, let her get what she wants.
Have you asked her what she would like? She probably has some idea. Just gently let her know that she really is allowed to have an opinion and that you would really like to know what she would like. Then over the weekend, go to the store with her and buy her some new underwear.
This isn't a big deal unless you make it a big deal. Let her know that this is perfectly ok and that she can have new underwear that makes her feel comfortable.