The guy is short(5'7") so he says the rear shock is at the lowest setting.

he either needs longer fork tubes to compensate for the 2" change in height, lowering links to put the rear lower and compensate for it, or he's just not going to experience a good ride at that speed.

checking the steering head bearings is a good idea regardless though. at least inspect for good condition and grease appropriately as needed. a steering damper would probably not hurt either.

I'm saying its the geometry. My SM has been lowered a couple inches, and it gets a little wiggly at high speeds. By putting on smaller wheels, the fork angle has been steepened, which makes it much "quicker" on turn in, but also more unstable. A lowering link in the back should fix it up. There should be a ton of lowering links for that bike. Also, a steering stabilizer would help. All these comments are assuming that nothing else is messed up, i.e. wheel, head bearings, etc.

you didn't say what he did for the rear wheel, what was the tire size change. ideally you want to raise or lower both ends roughly the same amount.

Just pull it up into a mad wheelie. Front end not on the ground = no front end stability problems. :D

Sorry, I have nothing to contribute but sympathy for a fellow 'tard.

He was looking at his bike last night and noticed that he had about 2 inches of thread on the shock so he is going to adjust the ride height. Cranking that shock spring tighter is going to make for a real stiff ride though.

I suggested he try to find a rear shock from a DRZ400 SM. I'm sure that is a shorter shock.

He'd want a shorter spring. He can however crank it down to see that it does cure the problem before going out and buying a shorter spring.

Most likely he screwed the trail up, making the bike unstable.

Tell him to check it using a rake and trail calculator

This one has more info
http://www.rbracing-rsr.com/rakeandtrail.html

But this one is a bit easier to use.
http://www.goldammercycle.com/hispeed/rakeandtrail.php

Don't tell me its cruiser/chopper shit. Its all about geometry.

Further for balance issues... go dyna beads and STU
http://lowdownchopshop.com/shopping-cart/14.html

He is going to get some lowing links and drop the ass end 2" to bring it back to the OEM rake.

put some nos energy drink in it