Are you dating this girl, or do you just want to date her? If you are dating her, I would tell her. But in a sugar-coated way. Like.....most of the time, it doesn't affect me, I just have to be very careful about what I eat and make sure I take my medications. She doesn't need to know anything else until you are getting serious (or until you have an attack while with her). If you just want to date her, I would tell her up front and give her all the details so that she knows what she is getting in to. Most people I know would not let it affect their decision on wether or not to be with someone they really like. Good luck and keep us posted.
And everyone I know-friends or not-that know I have UC are very supportive and don't care at all if I have to get up and leave to go to the bathroom, and they are not offened if I don't eat something.