Well, there's some variables here that you always need to consider with high school sports.
Is he suspended for the season, never to play again, or being sat down for one game? Was anyone arrested at the party, or did the cops come and break it up because the music was loud and the neighbors complained? Is it a fairly meaningless regular-season game or the playoffs?
Always got to keep in mind that we are dealing with teenagers who do stupid things from time to time. Not everything is worth breathlessly reporting on.
If he's being sat out for one non-conference game and he wasn't arrested or suspended from school, who cares? It's not going to affect the team going forward. It's the old "violation of team rules" and leave it at that.
If it's a situation where it needs to be reported -- an actual longer-term injury caused by jumping off a roof while drunk, a long suspension, a playoff series that's going to be heavily affected by his absence -- then you figure out a way to address it like any good reporter. We all learn of things off the record or in informal settings that we later work into more formal interviews. It's how you effectively work a beat.
A well-phrased question will get you your answers, or at least open the door and lead you down the road toward them.
In either case, you still ask the questions about why the kid didn't play. The difference is in how informed your questions are, and whether you can let the coach know he's full of shirt.