2008 Subaru Impreza jump start

I have a 1985 saab and it has a broken terminal on the positive cable and a few times after being turned off the battery seems dead. No lights or any noise. I just have to get a jump or hook up the charger and it will immediately fire up. On the last day it did this it was cold outside and I had a friend come over to give me a jump. He drives a subaru impreza. He hooked up the battery cables while his car was turned off and as soon as the cables were on both batteries my lights and everything came on so I started my vehicle which started right up and then he removed the cables and said "that probably just fried my alternator" meaning because I didnt wait to start my vehicle that it somehow caused problems with his alternator (on the host car a Subaru) He then got into his car and there was no problem it started up fine and two days later he said his alternator is fried. I believe he must have meant its going out because currently he is still driving around. The question I have because he now wants me to pay for it, is am I at fault. Would a jump like that even though it was cold out and I had a broken terminal, I started it right away after seeing all lights come on, is it because of the jump that the alternator is now going out