Glad you had a good intro session with your friend!
My own cautionary tale about getting over-eager to give a lesson to a friend - I tried giving a visiting friend a quick intro/lesson, and unfortunately it ended with the wheel slipping out forward, he couldn’t get his feet off the pedals in time, and him landing on his tailbone. Months later it’s still sore and I’m pretty sure he’ll never touch a uni again 
Broadly I think I was in too much of a rush and not well enough prepared. More specifically I made several mistakes:
I think the biggest one is that I should have spent more time helping get comfortable getting on and off the uni. I pretty much just had him step on and up and start going along a wall. When I learned I had a teacher who had me put one foot on and step over the uni a few times before even trying to get the 2nd foot on; in hindsight I think this is a pretty valuable drill to start with. Maybe stepping up and then back off would be a good one too. Probably also should have given a bit of a primer on bailing, including letting the uni itself drop.
At the time my smallest wheel was a 26, so that’s what I had him on. Maybe not a dealbreaker on its own, but definitely didn’t help.
I was having him go along a wall with one hand on the wall. I offered to let him hold my hand/arm with the other hand and he declined. He probably could have saved himself with some support on the other side; instead he tried to lean against the wall, twisted awkwardly, and went down. I don’t think support on both sides is strictly necessary starting out but it might have overcome the other factors in this case.