Warping and welding go hand in hand. What happened was the frame sprung on you when the pipe was knocked out. There may not have been much he could have done depending on the hardtail design and where the welds needed to be placed. The problem may not be that he rushed it, but that more welds needed to be placed on the inside of the tubes than on the outside. You should take it back to him and he should be able to to use a straight edge to determine where the tubing warped, and use a torch to apply heat on the opposite side which will pull the sides back out again. I'd be willing to bet that both sides start to bend in after the rearmost upper and lower braces that go between the two sides. Some heat applied just to the rear of those four welds (on the outside of the tubing) should bring it all back out. He'll (or you'll) probably need to use the cutting head of an oxy acetylene setup (without pressing the oxygen lever!!!) so he can localize the heat in small spots.
If none of that makes any sense, use a jack, pry bar, or whatever you need to use to push the sides apart to get the spacer back in. Then heat the tubing (with the rosebud tip) by the four welds that hold the upper and lower braces until they are red hot (heat to bright red, more than dull red, less than orange). Try to heat them all evenly if you can. Then let it all air cool. Once it's totally cool, pound the spacer out and the frame should spring much less.
Also, I would fix the frame vs machining the bracket.