So obviously drawing down is out - but if I'm carrying concealed and I jump on the guy, I've just doubled his chances for getting his hands on a gun. (It's not going to stay concealed for long while wresting.) From watching the video, I have a good example of keeping my gun hip away from the guy, but is it worth the extra risk? Especially since most civilian holsters are not exactly double - or triple - retention holsters.
And that's probably not something to include in the "casual" conversation while asking to help. (How can I help? By the way, I have a gun on my right hip, I have a black belt in, blah, blah, blah, president of the neighborhood watch, blah blah blah, memorized dragnet, blah blah blah.) I would rather the BG doesn't know I'm armed, if I could help it. But, I don't want any unwelcome surprises for you guys, either.
(Honestly, I'm not trying to chase the "what if" monkeys - I just want to actually be helpful in a situation like this, not the untrained do-gooder that inadvertently makes everything worse.)
If I'm in my own truck, while calling 911, that could be a chance to leave the gun behind, safely locked in the cab. Then it wouldn't be a worry.
I've not said that I never want a civilian to pull a gun out to help me -- but that the situations are few and far between. If you're carrying a concealed weapon (I'm not immediately aware of any concealable holsters that are high-level retention... the two kind of work at cross purposes.), the best thing that you might do is call it in, and then back off and take a position where you can act if the ogre gets my gun. There's really no hard rule -- but one of the scariest things professionally for me is covering a partner as he or she moves to cuff someone, because as they do so, I'm pointing my gun uncomfortably close to them if things get hairy.
There's no magic answer I can give, that if you do A then B, you'll be doing the right thing. You have to evaluate the situation, and do your best to make the best decision about what you can do. It's what I do just about every day I work, at least once... Sometimes, several times. And when everything is already FUBARed... it's even harder.
One thing that's pretty much never the right thing? Firing a warning shot. But... might the right thing be to run away? Yep... Imagine a well meaning civilian who decided to try to help during the infamous North Hollywood bank robbery shootout... Probably in the wrong place, huh? At the same time... maybe if just one person had grabbed the bad guy's arm in the video that started this thread, the fight would have been over in a few minutes. Hell... I've seen scary-big guys who were fighting like wild buffalo on 'roids get meek as a lamb when a female officer used "mommy voice!"