First, it's spelled "warranty". If you ever want anyone to take you seriously, learn to commuicate like you were not taught in a government school.

Second, depending on what it all looks like, it may or may not be covered. For example if he had a SRAM chainlink that broke, there's a recall on those. And subsequent damage would also be covered. But it's going to be hard to prove since he probably would not have the part.

Here's the issue: The OP is on his own. He'll have to ship the whole bike back to Mike for review, then be told yes or no. Then have it shipped back either repaired or not, depending. If not, then the OP gets to go off on his own and get the problem fixed. Hopefully, you are riding by Thanksgiving.

In the LBS world, you bring it to the shop. If it was me, even if not under warranty, I'd sell you a derailleur at cost just to keep you as a customer. And you wouldn't have to wait for a month or so for some unseen person to look at your bike, where since they don't have to look you in the eye it's a heckuva lot easier to say "no" and send you on your way with a shipping bill.

Have fun.

This is the cost of doing business.

If it was me, I'd just go to a shop and get it fixed unless you want to mess around with this for a month or two.

A WARRANTY is for MANUFACTURER'S DEFECTS. Not for JRA's. That can only be determined by the manufacturer or their representative.