What's the background for the wedding - is it a blingtastic party or a formal religious ceremony?
As a general rule of thumb, I'd say, if in doubt, cover cleavage, shoulders and knees. If it's at a mosque, there are sometimes rules about taking shoes off - in which case, you might want to think about your preferences re: tights, pedicure, nail polish, etc. Also, watch out for shoe thieves! (Yes, it's a thing...)
If you have outfits you want feedback on, there's also the Muslim Mumsnetters board, where we'd be happy to try to help there too.
Remember that your friend has invited you, and will just be happy to have you there - you don't have to pretend to be someone or something you're not.