Just thinking out loud. If you measure the pitch of one tooth on the belt, that is how much the axle will move with a 2 tooth smaller rear pulley. Take a look at your pal's bike and see if the axle can move that far and not hit problems. Frankly I think you'll be lucky!

 

Ok sounds good I will measure it that way.

If going w/ sporty pulley what year has a 1-1/8" belt 68 tooth? I bet the dyna pulley will fit.

 

Here is a pic of the badass: