Is it rude to tell my gf not to drink too many cans of monster when I drink twice as many
Why would you tell her not to drink them?
I've seen her drink 2 cans while I've drunk 4 times that number
It's doesn't make sense to tell someone what to do while you're doing the same thing. Maybe it's best to focus on yourself and let your girlfriend do her thing.
It's going to be an April fool's joke.
I'm not sure she's going to see the humor in the joke after watching you guzzle twice as many as her. It's probably going to come off as just rude. I'd advise you to think of a different joke to play.