Most 3.5ers have header type connectors, not card
edge. I think he's going to have to snip the wire on
that particular cable. Unless of course he's using one
of those little adapter thingees with an available
card edge connector on the end of his cable. That
would be preferred.
I doubt you can still get them, but I've seen those adapters (heck, I've
got a couple in use on my 5160 machine) that were in the form of a small
PCB with card edge fingers (to plug the cable connector onto) and a 2-row
34 pin harder socket soldered to the PCB that fitted onto the back of the
3.5" drive. And there was a jumper on the PCB labelled 'XT' and 'AT'
disconnected pin 34 inthe 'XT' position.