I dunno, if your friend said she is not "butt uggly", I think she meant she is just average to ugly. If you do it don't expect a hot girl since if this was the case she would be able to get a date in 5 seconds (All of this only if you are shallow but since you said you weren't it shouldn't matter). I, myself, was asked by my sister's friend (who is actually pretty hot) but I politely declined. For me, it would just feel weird to be back in high school and even weirder to be back at prom having been there 3 years ago. In other words for me the age difference is small (as long as she is 18 or older) but the fact that it's a HS Prom is what sucks.
Regards
ng