Was it stone dead when you left it for him to jump? Have you tried jumping it yourself since finding the cables like that?
Before I replaced my alternator I had a current drain problem and would have to jump my CJ every few days if I didn't drive it. The cables would get warm/hot like you describe. I replaced the cables in the jeep first to make sure they weren't corroded inside the jacket and causing excessive resistance but that didn't solve the problem. Turns out it was crappy jumper cables. They were the discount parts store kind that only have the wire crimped into the clamp and mine had the insulation crimped into the connection as well.
I unfolded the crimps, stripped back the insulation, re-crimped and soldered the connections and it was a night and day difference.
'84 CJ-7: Yota axles (spartan locked front, e-locked rear) 4" BDS, 1" Daystar & 3/8" shackle lifts, 35" X-Terrains on steelies, YJ Tub & family roll bar, heater blower upgrade, 4.2L w/MC2100 & Team Rush, Warn 8274. Rebuilt 4.2/4.0 hybrid, AX15 and twin sticks/clocking ring waiting to go in.
The advertised position for a topless, dirty jeep girl has been filled......by my wife. Thanks to all that applied.
<------------and yes, that is her. :-D