I'm going to upset my friend's gf
What could possibly be the reason why you want to upset your friends girlfriend?
I just really dislike her.
Someone else's is significant other may seem annoying for me too, but you shouldn't go around upsetting people just because you dislike them. If someone else had done that to you, it wouldn't feel very welcome nor accepted as well, right? Try to get to know her, or maybe just avoid her if you really dislike her instead of upsetting her.
I'm doing it it's just a joke
Well she might get really offended and not find it very funny. I would be really careful about this situation. Even if you don't like her do you think you could try to keep the peace?