I have heard many times how someone blames someone elses hard drive for fouling theirs up.
Firstly, if you are going to install a suspect drive into a system, always put it onto a different channel to yours. He should have put your drive onto the secondary ide channel and not slaved it off the primary, but maybe he did?
If your drive is still working and giving no problems then no way did it cause his drive to pack up, his drive was going to pack up when it did regardless. I tend to leave my system running 24/7 and its only when I eventually turn off and on again (about once a month) that a hard drive fails. This is when you get a 'surge' of power into the drive and any problems with bearings will come into play as they have to try to spin up and become balanced again. It may have been that your hard drive had some kind of boot sector virus? which found its way to his hard drive. Did he try to reformat his old drive to see if it was ok?