Sometimes people turning wrenches just don't get a feel for tightness. For example, removing an oil filter from a car should not take a boatload of special tools, but I have them in spite of that.
If you can get the post off, they can simply send you a new one.
Lay the keg down so you can put your knee on it. Have the fitting on the opposite side so that the leverage of the wrench is across the center of the top of the keg. You can even put a long pipe or rod through the openings of the rubber part to steady the keg so your wrench doesn't turn the whole thing.
Did they respond to your email yet?