It's a cheapo, mass produced MTB with all entry level running gear that is used exclusively on the road. The QR wheel spindle snapped on him one day.
The shop replaced it with a solid one under warranty which has now also snapped (bike is out of warranty now). He asked me to have a look at it. As far as I can make out, the wheel is in square (it's snugly into both dropouts). What I did notice is that he had it on the smallest chainring and on the smallest rear sprocket. When I asked him, he says he does that quite a lot.
It has an 7 speed rear hub and it isn't exactly compact so that makes the chainline pretty far out.
My theory is that stomping on the pedal with the chain in that position must be putting a fair bit of twisting force on the back end of the bike. Reckon this could be the reason for the snapped axles?
I've told him to avoid the big-big and small-small gear combinations in any case.