That story reminds me of this party I went to bout 6-7 years ago in high school.
Party was going great, everyone had their drink on and a few joints had passed around when a few kids from the next town over show up. They ended up not being able to hold their alcohol at all and started getting all pissy so me and some bros decide we'd put em in their place.
Beat two of them pretty bloody and one of the others passes out like the pussy he is so we're thinking of ways to make his night even worse. Somebody said I should take a piss on him and I'm a few too many in to say no, so I drag him into the washroom to keep the mess to a minimum.
I was still young enough to get those random boners that happen for no reason at all, and sure enough when I get my dick out to piss on him I'm rock hard. That won't do, so I decide to wait a couple minutes for it to subside. Two minutes later I'm still rock hard and think 'well I might as well just wack this beast down' so start tugging for a bit.
The dude was curled up facing away from me actually had a pretty nice ass for a dude. I kept going a bit and the more I looked at it the nicer his ass was. Like I said, at this point I'm drunk enough to think anything is a good idea so as soon as the idea popped into my mind there was no turning back.
I grabbed some shampoo from the shower, pulled the dudes pants down, lubed up a bit with the soap and gently pushed my cock into his asshole. Damn it felt good. So tight.
I was going for a minute or two before some asshole opens the door and starts yelling at people to come see. I can't finish now but still have this boner, so I decide to just walk it off instead so take a few casual laps around the party to get rid of it.
Was a pretty good night. Sorta sounds like what happened to your buddy.