you will be fine drinking once on an oral.... it may temporarily raise your liver enzymes.. and you will more than likely have a pretty good hangover...lol
but, drink a ton of water for the days leading up, double up on your liver supps(for a few days prior and a week to two after), as well as drinking tons of water!!! if you REALLY wanna get the enzymes back down quickly, avoid fatty food at all for a couple days following(and take extra efa's)... and you will have any elevation in enzymes back down very quickly....
I used to drink a hell of a lot more than I should while on orals(back in the day).. not the smartest thing to do... but, I did take tons of other precautions(like what I named above)... my liver actually, never got outta hand..(of course I was young too... I would never advise that sort of thing)...
have fun.. it only happens once a year(his bday).. and you only do it a couple times a year.... just make sure you have transportation(that, honestly, is more likely to end up having detrimental outcome than your one drinking experience on the oral).... and, if you wanna take a little extra precaution, halt the drol for two days before and after the drinking, while taking the other precautions.. you may not get any elevation at all in enzymes....