Without looking at it there's no way to know what caused it other than oil starvation or oil pump failure. I do think that they must have left it on tickover unattended though, oil starvation and temps reaching too high may be the cause, but you can't prove that they weren't paying enough attention to it to stop it in time to save it from permanent damage.
Personally unless you can find out the cause and also determine it was their fault I'd see if they can do something to replace the engine at a reasonable price that you are both happy with which keeps it all friendly etc.
If they turn around and are cocks about it then you'll see their true colours and realize that they probably shouldn't have been used in the first place, which unfortunately we are all unaware of unless we've used them before.
It's worse for mechanics to go to unknown garages because we see the signs earlier on and if we feel that we have to use the garage it gets depressing
You are also entitled to take the van elsewhere, don't be fooled into thinking well it's here now and it'll be cheaper than taking it away for repair because the garage will also know this and may charge more so get quotes!
Also if an external component failed then you need to fix that too or the new engine will go too!