This happened when I was about 18 years old.
Growing up, I had a friend named Steven, who I hung out with all the time. We were best friends. We talked about all sorts of stuff together, sports, girls, sex, masturbation, etc.
One night during the summer, we had been sitting on his porch as usual, just talking when the conversation led to masturbation. I was curious about some things so I started asking him some questions.
I said, 'I know this is going to sound weird, but when you squirt, how far does it usually go?'
'Oh it just depends, I've shot it up to my chin once, but usually just a foot or so. You?' I told him that mine usually only goes a few inches. I'm actually mostly a dribbler letting it run down my shaft and pool up on my scrotum between my balls.
Then I asked where he usually did it. He told me that he usually did it in the shower, but he also liked to do it in bed with a special sock he kept behind his dresser. I told him I usually just do it in the shower.
Anyway, the conversation went on for awhile, and we finally decided to go up to bed because it was really late.
He slept on a mattress on the ground, and I would sleep on the floor next to him, as always. We talked a little longer and finally got quiet to go to sleep.
I couldn't really sleep because I was so horny from thinking about jacking off due to our conversation. So we laid there in silence for awhile, until I heard Steven say, 'Sean?' I didn't answer because I didn't really feel like talking.
Again, a little louder, 'Sean?' I didn't answer. Suddenly, I heard him roll over then lay back down. Then, I was caught completely by surprise.
He started masturbating. And I don't just mean slowly rubbing one out. I mean jerking like there was no tomorrow. I could hear him start breathing heavily and his hand rhythmically running up and down his shaft. Then after about five minutes of this, he let out a long sigh, and the rubbing stopped. He rolled back over and put what I assume to have been his sock back behind his dresser.
At this point I felt like I was about to explode. I waited for a bit, then got up to go to the bathroom. As soon as I got into the bathroom, I jerked my boxers down, exposing my raging hardon and started stroking. I was so horny that I exploded in about 30 seconds of stroking. It shot all over the mirror and sink, so I had to clean it all up, then went back to bed.
This happened many more times whenever I would stay the night with Steven, because it was his regular routine aparently.