Based on your description it sounds like a clutch cable issue...
But, I would dismantle the lever, clean, lube and adjust it first. IE: if you or your buddy starting tightening some bolts, you may have the lever itself too tight. Also, check the lower end lever assembly. On the 2G 600 this is on the countershaft sprocket cover. If it is bent or clogged with dirt/chain lube this can also be your problem. If you disconnect the clutch cable at both ends, you can determine if it is the cable. You should have almost NO drag on a good cable.
Your going to have to do this work to replace the cable anyway, so cheaper to do it first.