Someone who works in your building, who you only see on visits to/from the restroom. You have no idea what their name is or what their specific job entails, so exchanges are limited to non-specific pleasantries (e.g. the weather or local sports teams).
A: "Did you see that douche in the paisley shirt today?"
B: "Oh yeah. He's my bathroom buddy. Our bladders are on the same schedule".
A: "You should've heard the fart he let out standing at the urinal"
1) Bathroom buddy can be a "thing" you bring to your bathroom to occupy your time while using the can.
2) A person that you only see when you go to the bathroom, meaning they go to the bathroom same time as you do when at work or at school.
3) A friend you bring along when you don't want to go into the bathroom alone.
1) "Every time I go to the bathroom, I bring my DS so that I can do something rather than read, while on the toilet."
2) A: "Great, Joel has to go to the bathroom. How much do you wanna bet that Adrian is going to be in there, too?"
B: "Well, that's not a fair bet, Adrian and Joel are practically bathroom buddies!"
3) A: "Hey, Adrian, can you tag along with me into the bathroom? I feel weird going into it alone in public."
B: "Uhh...So you need me as a bathroom buddy?"
A: "Yeah, if you don't mind."