I once saw a little Muslim girl dressed up as Hatsune Miku, except instead of a wig she used a teal colored Hijab and incorporated the colors and patterns of the costume into a traditional appropriate outfit.
Hijab Hatsune Miku
Hijab(I think) Lolita
And if you just google image search 'Hijab cosplay" you can see a lot of people/concepts that are really cool
Disney Princesses with Hijabs
So I hope this helps and maybe shows you and your friend that she can cosplay anyone she wants and still find ways to make it appropriate to her culture
Personally I think it can be really cool and a unique way to portray a character!