We're getting married next year and we both have small families with very few men! My brother has been asked to be an usher but he's the only one at the moment and we're concerned that we may need at least another usher to help out (we're having 60 at the ceremony so guess that just the one would not be enough). Could we ask a female friend to be an usher?
It's perfectly acceptable to have a female usher. I'm sure she'll be thrilled!
With regards to her outfit, while men are happy to wear a dark suit, women like to dress up a bit and she probably had planned to wear something special for the big day. Why not suggest to her that you are happy for her to wear whatever she would have chosen anyway (after all, what better excuse to splash out on a new outfit!), but you will have a special corsage made up for her which will make her stand out from the other guests. I'm sure she would be happy with this, and if you can afford it, then by all means offer to contribute to her outfit.