You've got it wilwren -- when it'* somebody else'* car, your options are severly limited. If it were mine, I'd just start drilling and modifying electronics until I was satisfied.
I've recommended the next step for him is a full coolant backflush, and if that doesn't solve the problem, a new radiator. With the A/C on, those fans are pulling the heat from the condensor through the radiator. If the radiator isn't as efficient as it should be, the heat from the condensor is overwhelming the radiator, which leads to overheating.
We'll see where this one goes, I'll probably see him in a week or two for the same issue.
Thanks for all your help, and if you have any more suggestions, post 'em!
'94 Bonneville SLE | 266K miles
CAI - K&N cone filter - polished intake runners - 140 amp alternator - 2.5" exhaust - Magnaflow hi-flow cat - Magnaflow dual outlet muffler - 106K on engine and transmission - Sold 1/14/07
94 VW Jetta - 151K - 2.0 - 5 speed - Ghetto black steelie wheel mod - Techtonics 2.25" cat-back system