: The cables that bicycles shops sell is stainless and can be purchased off a
: roll with or without the sleeve.

Bicycle cables have a casting on one end for the brake or shift lever. They are sold in "universal" lengths, but at ~7', they're way too short for a kayak rudder cable. I've never bicycle cable them on a roll, since the end fittings are required. The housing comes on rolls, as the end fittings for it are simply slipped on.

: I'm 99.99999 % sure on the stainless but ask to make sure.

Most are, some aren't. He'd probably be better off to buy it at a marine supplier, where he can be sure of what he's getting.