Welcome to the forum.
First off, the standard disclaimer advice it to tell your friend to hire a pro. A wedding is a one time event (or is supposed to be) and things could become pretty bitter if "the photos didn't turn out". Back up equipment is very important...have two (or more) of everything.
That being said, I would get a flash. You could go with a fast lens & fast film but then you have grain to put up with. Maybe if you could check out the hall, you could test out what light levels you will be working with. If you do get a flash, test it out beforehand...a wedding is no place to be testing out equipment for the first time.
I would stay away from slide film. The exposure latitude is quite narrow...best bet is color negative film or B&W if you like. You might want to use a pro film that will help get good skin tones and decent contrast between dark suits & white dresses. I think Fuji NPH is often recommended (try a search).