If you had the snakes for 2 weeks, it's difficult to say they "developed" RI in those two weeks from something they had prior to shipping. Normally(not always, of course) shipping brings any weaknesses or illnesses out in a snake. But if they took two weeks, that's awfully iffy.
If Mohammad offered to pay the vet bills on the snakes HE shipped to you, that's really nice of him. But if you chose to keep substandard snakes, and he said you could return them, you assume all liability for care. If he did not say you could return them(at the time of shipping, I understand he said no return NOW), then he should pay vet bills.
And yes, quarentine ALWAYS. If you're asking him to pay vet bills on animals he did not send to you(your animals which were exposed to the new animals) then that seems a bit unreasonable. You should always practice some quarentine so that this sort of thing doesn't happen, although I understand hindsight is always 20/20.
I quarentine EVERYTHING, including snakes from people I trust totally. I put one of my OWN animals into quarentine because it came into contact with a new animal. Paranoid? Maybe. Safer? Definitely.
Good luck with the ill snakes. I hope they turn around for you and get better.