That's probably a five-minute, $5 fix, unless you've rounded out the hole in the crankarm. Then it's a five-minute, $150 fix. I'd mention the warranty angle, on the basis that a crankbolt that was properly torqued when it left the shop wouldn't have worked loose, but I also understand the shop's probable defense: If you rode the bike after you realized the crankarm was loose, everything that happened after that was on you. Then you say, "But I was far from home" and they say "if you'd done proper maintenance, it wouldn't have come loose" and everybody's unhappy.
But I still say a bike that was assembled right when it left the shop wouldn't lose a crankarm.