Do not yell at a fat person to lift with their knees when their gut prevents them from getting into the right stance. Give them a lighter object, and motivate them to keep working out.
Seriously. Strap a giant beach ball, half filled with water, around your midsection, and try to life with your knees. You will quickly figure out that you are being both an ass and frustrating the person trying to lift.
This happened enough in the past that the last time I just dropped the crate and walked away. When the group lead confronted me, I explained it as above, and even explained that I tried to get the other guy to understand that it just wasn’t going to happen.
The lead when to the other guy and chewed him out so badly that he left the volunteer site. I ended up carrying the crate on my own.
All that stupid fuss over a crate that was 3′ 8″ long and 25lbs, mostly empty. I hoisted it onto my good shoulder and soloed it into the shelving unit. Job done, and one idiot removed.
[FTR: I waited a couple of years to post this, since I didn’t want more fallout against the guys that I was helping.]
Bottom line: Take a serious analysis of those you are working with. Realize what they can and cannot do. Capitalize on what they can do. Don’t punish what they cannot do.