I'm not a pro mechanic anymore, but I do turn wrenches on other peoples's bikes from time to time. I always try to give it a test ride on it's own wheels, not just on the stand, to make sure everything is OK before I give it back. Most of the shops I've been in do this too. Time or weather may have prevented it in this case though.
Bring the pictures back when you go to talk to them, esp, the ones that show the busted link, and that first one, where it looks like the chain is over the tab.
That way, they know that you know the condition / routing of the chain when it happened. Important, since the bike is back at the shop now.
Be cool, and give them the chance to own up to it first and make it right.