Any Dana 35 or Dana 44 from a 1997 to 2006 Wrangler TJ will bolt right in. If you never plan to install a locker or do tough offroad trails, it wouldn't cost much to get your Dana 35 rebuilt or buy a used Dana 35 from someone who upgraded. Caution that his Dana 35 has 4.11 gearing in it so if you buy a used Dana 35, it needs to also have 4.11 gearing in it. It'd be $600 or so to regear it to 4.11 if the axle had a different ratio like 3.07 or 3.73.
If he wants to offroad his TJ regularly on tougher trails, this would be a good time to replace it with a Dana 44. I paid $800 for my used Dana 44 which bolted right up & was trouble-free on some even extremely difficult trails.
I personally would not go the Ford 8.8 axle route. They have a much bigger pumpkin which means they have less ground clearance than a Dana 44 does, plus it's not inexpensive to buy one or have it set up properly so it can bolt into a Wrangler TJ.
Personally, even if my wheeling was fairly mild, I'd rather spend the $$$ now to upgrade to a Dana 44 than to rebuild the Dana 35.
Just remember that whatever axle gets installed into the rear, its ratio has to match the front axle's ratio... his should be 4.11 if his TJ has the usual 5-speed manual transmission.