I agree...I don't think it's just "he's totally uncontrollable"...
I think your friend was a direct cause of the incident, and shame on him for not listening to you in the first place. However, that is past and now you have to deal with the 'aftermath'.
When he get's 'uncontrollable' have you ever just pushed him into a run (which it sounds like he might just want to do) and made him run until he is 'begging' you to stop?
I had a friend a few years back who had a mare who was like Blu; she was uncontrollable, especially if you put her into a trot and then a lope. Once she was loping, it was all over...you better just be prepared to hang on, until she wanted to settle down. So one night, when I wound up riding her, I asked for a trot, but she right away went into a canter; and rather than try to stop, or slow her down, I grabbed one rein, and pulled her into a circle. Eventually I was able to switch directions, but I waited until she was getting a bit tired, to ask for the change of directions. And I just kept her going till a little beyond the point where she was begging me to let her stop. She would try to trot, but I would push her right back into a canter. Finally, when I straightened her out, and asked for a stop, she stopped very willingly; and I asked for that trot again, and she gave it willingly without trying to run out on me. Might be something you could try with him.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."