I'm no grand expert, or boat failure expert for that matter, but my first impression is that I would have been surprised if that boat didn't come back with some sort of failure. It appears to be well worn, to say the least. The failure photo seems to show plenty of wear nearby (when and where that wear happened is anybody's guess) but the threads in the fabric seem off colored to me. Don't know what kind of boat it is, nor how long they were exposed to water but it certainly wouldn't be beyond conceivable that they had been getting wet and rotted. If the rental company is blaming you I would cry BS. From what the pictures indicate to me, the failure is just part of the life a raft and the rental company should offer their apologies for renting worn out crap, not asking you to finance their fleet maintenance.
All that said if you overinflated it then slammed it into a sharp rock then things could be on your head. Only you know that for sure, but to me pinning seam failure on a customer is at best questionable...
Yesterday's gone on down the river and you can't get it back. - Agustus McCrae