Timing belt broke, need advise.
 

So I'm new to posting on these forums, however I've done quite a bit of reading here over the past few months. I was hoping those with more knowledge and experience could give their opinions on an issue. I'm not very car savvy so I hope the following makes sense:

The head gasket on our '03 S80 T6 cracked and we picked it up approximately two weeks ago. They machined the head, did a valve job, replaced the alternator, and a few other miscellaneous things. Car runs fairly smooth, but when accelerating quickly it isn't as smooth as it was prior to the head gasket cracking (only throwing that in just in case its relevant to someone).

Last week we started noticing a scrubbing noise coming from the passenger side front tire. We take off the wheel to do a brake job and surprise, there is a 8-10" piece of housing behind the rotor near the ball joint. A quick Google search tells me this is the "Timing cover belt rail" (part # 9125469). Since it's Friday evening and the shop is closed we just set it to the side with the intent to call the shop on Monday to inquire about what it is, and why it is out of place.

Sunday my husband goes to take his mother for a leisure ride through the mountains when suddenly the engine makes a crunching gravel type sound and seizes. After popping the hood we saw that the sepertine belt had started shredding so we had it towed 28 miles to the shop that had done the engine work.

I called this morning and spoke with him about the car. The mechanic says the serpentine belt shredded, got stuck in the crank pulley, then made its way into the timing belt and broke it, which as we all know is catastrophic to the engine.

So here's what I'm trying to determine: Is this just an "act of God" like the mechanic is trying to tell me? It just doesn't make sense to me that he basically rebuilds the engine, replaces the alternator, and two weeks out the car is in need of a massive repair again. He's a very nice guy...so I'm not trying to screw him over but this just isn't making sense to me. Also how would that housing have ended up in the wheel well? Did it fall off and cause a domino effect? These are the questions running through my mind.
Edit: The car had also developed a squeak over the past 3-4 days (not a continuous squeal, just a timed squeak).Not sure if thats relevant.

Oh and I failed to mention that the work carries a year warranty.

I find it hard to believe it is just an "act of God". Maybe he put an incorrect alternator in. Maybe he left something loose. Maybe he put the crank pulley on backwards. Maybe the belt wasn't on all the way. Something shredded that belt.

That was my thought. I know little about cars but I didn't think that the serpentine belt should've just started shredding when it was fine a couple weeks ago. Something had to cause it.

They put the belt on too tight but also didn't tighten up the timing belt cover. Or the serp belt knocked the cover off (actually that sounds more like it). So it was one in a million but yeah, they need to take some responsibility I think.

You can't put the serpentine belt on too tight. The tensioner is automatic. But they could have had it not fully installed or put the crank pulley on backwards.