What gas valve are you talking about? Is there any relation to the EGR Valve that was replaced?
Sometimes when working on vehicles with rusted bolts, this can happen. It's very unfortunate, and ideally when you see that this about to happen you try to stop and let the customer know before something breaks, but that's not always possible. Without seeing the vehicle and assessing the situation, it's hard to know if this was due to negligence or not. I would get a second opinion from another shop, have them look at the repair orders, see what was done, then have them look at the vehicle and see what they say.