I'm not going to do my friends hair for an important night out with her bf. She should go to someone else for help.
Why don't you want to help your friend out?
I have my own reasons.
This is a very important date for her, so you are being a little rude for refusing to help her. She went to you to ask for help, so she acknowledges your skills and thinks you're good enough to take care of something so important. You need to keep being a good friend to her and just do it.
If I do it I might ruin out friendship...
Why would you lose your friendship? Are you afraid that you might mess up?