Needless to say, he didn't get picked for a scene again, not for several weeks.
Improv is often funny, and there are people who are naturally funny and also good at improvisation. But that's not the point. Improvisation isn't inherently funny - some of my troupe's most memorable scenes were drop-dead serious. (Literally - one was about the death of a character's mother, reflecting the improviser's own current struggle with a parent's major illness.) The funny scenes in improv arise out of naturally occurring situations that don't quite fit. When we listen to our scene partners on stage and try to add the next logical step, that's where it often gets funny - because the first "logical" thing that comes to mind might not be expected or normal or even logical. But when improvisers accept it as the truth of the scene, and play it forward with 100% honesty, that's when the magic happens.
Sometimes that honesty will get us in trouble, so to speak. While as a person, I might not really want to slip on a banana peel for fear of embarrassment or injury, as an improviser that's exactly what I want to do because it's far more interesting. Dynamic aspects of a scene like status changes, or getting knocked down so the audience can root for you to climb back up, make much more enjoyable scenes and give the players so much more material to work with.
Unless your scene happens to be in a fruit-peeling facility where banana peels on the floor are commonplace, that banana peel is there for a specific reason. A good improviser will choose to be that reason.