I'm going to tell my best friends girlfriend that he messages random girls
What is the basis of your thinking? How would that make you feel?
I would be proud of myself for telling the truth
You normally shouldn't meddle in your friend's relationships, but if you think that he is cheating, or otherwise being dishonest, you might be justified.  But try to talk to your friend alone, first, and get him to stop before you tell his girlfriend.  You wouldn't want to break up a relationship without giving him a chance to improve.
I have tried to hold myself back but I can't wait anymore.
If your friend is behaving inappropriately you should try to talk to them about it before making it a public issue.