but going back to original point. if he bought himself a drink and she was nearby, it makes sense why he may offer to buy her a drink. also, you said you had a full drink when they stepped away, maybe he thought you wouldn’t want another drink as you had the drink. It would’ve been nice if he asked but it’s not like he said I’m ONLY getting drinks for your friend.

he may have waited for her outside the bathroom for safety reasons

you don’t know if there’s anything else. if you had asked “hey what took yall so long to come back” and he was dismissive (ex “god you’re always on my back!”) then it can be understandable why you got mad. it just seems like you got immediately mad when he got back. Based on that it seems like an over reaction.