I remember a few years back some guy had a couple of bolts break and BQ explained to us all that that the bad left slot was the cause. As a matter of fact he said that you could keep on putting new bolts in and they would keep on breaking. That receiver has only one shot at being salvagable and that would be to weld up a fix and have a good machinist try and save it. People forget that some of these carbines were run as M2s and God knows how much that beats them up. The owner has got to understand that the carbine needs a couple of things to work well, cases that are trimmed correctly and a bolt that closes correctly with a good fit on both sides. I would suspect that that receiver was already messed up and when the bolt let go it was REALLY messed up. Because the left slot is just a guide the repair might survive well in spite of not being reheat treated.
Take a look an an AK47 bolt and slot sometime. The left hand lug will not even be touching the wall of the slot while the right side will be fit perfectly. With the carbine, if there is anything that limits the touching surface of that right lug, it really puts a strain on an already fragile bolt. A banged up left lug that sits slightly crooked in its slot will doom the right lug unless you have an exceptional one and I don't think that there are that many perfect ones.