Can I tell my friend that his gf can't bring her friend on our trip
That depends. Why do you think his gf can't bring her friend on the trip?
He wants me to tell her that she can't so that he doesn't have to pay for her too.
I understand that you may be concerned about fairness, but getting involved in your friend and his girlfriends business is inappropriate. Go on your trip but stay out of bringing up their issues.
I can just see myself bringing it up, it's fucking hilarious how mad they would be.
While you might find it funny, getting involved would only cause them to get upset at each other. Your friend might even get mad at you too. This could make the trip end up going poorly for everyone.