My opinion:
2 ways of looking at this. First, is that I believe Superwinch has one of (if not THE) lowest amp draws of any winches, so any decent sized jumper cables should work fine, especially if he doesnt run the winch at a high duty cycle for heavy loads. Pulse the winch, second or 2 on and off etc.
Second, is that a bunch of people end up running extra accessories, dual battery etc. Myself, I ran 4ga cable up under the hood to a buss bar point, then ran the winch's cabling off that. If there's any plans at all in the future of a 2nd battery, or extra accessories, or running the winch very heavily, I'd strongly push towards larger cable. Do it right the first time and not have to worry about it as much later on.
Either way should work fine, just depends on what the future plans are. Get the machine up on some ramps, few bolts to take the skidplate off, ziptie the cabling to frame or whatevers you can find underneath and up into the hood, doesnt take very long.
Edit to add: but yes, please don't do a splice, make it 1 cable run.