Hi Horsef, All!
Once he settled down, I see no problem with picking up his reins/lead for his owner as long as you are confident enough to deal with the explosion that might happen when you attempt to do so. As long as he remains agitated (big eyes, pinned ears, rapid breathing, snorting, etc.), your best bet would be to stay out of his way, maybe even dismount and exit the arena if you can do so safely and not let The Monster out.
Worst case scenario, do whatever it takes to get a fence between yourself and the animals. Realistically, this should probably be your first reaction; get to the fence/gate and prepare to bail out. Then if the situation allows it, evaluate per the above, and take whatever action seems prudent.
Steve Jernigan KG0MB
University of Colorado