I just repaired a '93 850 last week where the customer failed to ever replace the timing belt so at 104K miles, it broke.......$3K bill total.
If your belt was recently replaced, you need to determine the cause of why it failed before trying to accuse anyone. Was it a Volvo OEM belt or a cheap aftermarket belt? They're not "all the same". Did the t-belt tensioner fail (possible) or might there be a bad pulley that could have caused this?
No matter what caused the 2 month old belt to fail, I'd doubt that it was the mechanics fault unless you can prove that he failed to notice an obviously bad, related part or that perhaps he failed to tighten the tensioner and its bolts backed out. If that did happen (and you'll need another qualified tech to know if that's what did happen), THEN you might have a legal chance of proving poor workmanship and his shop may be liable. On the other hand, if everything seems OK yet an aftermarket belt just broke for no obvious reason, winning that battle could be more difficult BUT possible with a lawyer.