cam timing different bank to bank
 

What are the effects of engine performance on a 2L engine with webers if the cam timing on one bank is set to the minimum of the spec and the other bank set to maximum of the spec? Would one bank run hotter than the other because the ignition is more advanced on the one side?

Your ignition timing is locked to the crankshaft (and therefore piston position), not the cams, so ignition timing would be the same for both banks. What would differ is the way the engine breaths on each bank. One side would have more torque in the low end, and the other would have more power in the top end. What you end up with is 3 cylinders weaker than the other 3 in the lower and upper ranges of RPM, which makes for a less efficient engine.

My friend's 2L engine is running with one bank 30 degree hotter than the other bank. I surmise that cam timing on one side is more advanced than the other. Is that a possible cause, and if not, what else could be the cause?

If it has carbs check the float bowl levels and see if the fuel level is lower and therefore running leaner in the hotter bank.
Or maybe there's an old rodent nest under the air shroud on one side or the engine fan swallowed a rag and blew it over on one bank and that side is getting less air cooling now.