I'm going to call out my gf's friend for going in our room whilst away
Why do you feel the need to call her out?
Because I want to know if he's planning anything with my girlfriend
I understand your suspicions.  Just make sure to do it in as respectful a manner as possible, because starting an argument with a partner's friend can lead to some relationship issues if you're disrespectful to the friend.  I hope you get this resolved amicably!
I sure did and what is respectful about calling someone out. It's inappropriate to do.
Exactly, I think that you have to be tactful in the way that you do this. It it turns into a giant argument, you can also make your partner mad at you as well.