Thanks Angela, and also thanks for defining "bag of spanners" since I only had the vaguest idea what it meant.
Just to throw in a older married guys percpective into this situation though, I don't see the mention of the mate's age as an excuse (because it is about as weak as they get) but a way of explaining why the situation occured, maturity being the other relevant factor.
Young men can have hard time saying no to lil' Elvis, especially when fulled by alchohol, because they don't have experience dealing with the consequences of their actions. No matter what advise you give your mate (or yourself, if applicable) there will be consequences. The girlfriend will find out about it one way or another, the worst possible way being from someone other than the person who cheated. Hearts will be broken, the circle of friends will discuss why it happened. Your mate will have a hard time on the rebound because any potential new girlfriend will be repulsed by the back alley shagging (wasn't there at least a car back seat available?)
Your mate and the girlfriend will hopefully emerge from this with no STDs and wiser for the experience.