I want a date in order to go to a function but I'm afraid to ask my friend because he has a girlfriend. I'm afraid people with start to say things, like saying that I'm taking the guy out when he has his girlfriend. Do u think I should just ask him and forget about what other people will think?

Ask him and let him decide if it's OK. No secrecy, though. He should tell his girlfriend. If they are both OK with it, then who cares what anyone else thinks. You've been upfront and honest with him and his girlfriend. You don't really need to explain this to others either.