One of my best mates, a girl, me and a party

Opinion and advice all in one here.
You were sober (or close to as stated) kissing this girl that he was "seeing" not actually in a relationship with is still wrong. And in the same bed, jesus.
I wouldn't take this thing any futher until the girl tells your mate she doesn't like him, you talk to your mate about it and it settles down a bit.
She sounds like a bit of a player to me, kissing you while the guy was next to her. It's exciting for all the wrong reasons, hell look I've done it. Not in a bed but in a club and knew a guy who liked me was watching. He wasn't best pleased with his mate and I didn't really care. Oh to be young and lapping up the attention
So think about it, she probabaly doesn't even care you are good mates and if she does that to him isn't she likely to do something just has mean to you?
Bros before hoes always.

the issue is whether your mate was aware that you were in the race for this girl.

whether the rules of fair play were in effect or did he asume you'd let him have a go?

to round it up... were your intentions towards the girl clear from the start?

if the answer to any of these questions is no then the 'bros before hoes' clause applies.

simply explain if they didnt pull all is fair in love and war (within reason). if they did pull back off, warn him off her, and go out on the pull together and get new mefanwe! that is the best situation. you be the wingman, set him up with someone nice to say sorry!

thing is i dont wanna make a more of a prat of myself (little chance of me doing that in his eyes probably) and dont want to sound harsh on him. eg.

"Hiya mate, did yu reeli pull her or is that just a lad of ******?"

and dont want to just ignore the issua and pretend everythings fine.

your last sentence sounds like a plan

My diagnosis for the op - TOPGUN!!!!!!! that will answer everything. it always does......

topless volleyball! wooop!