First question, is it your bike that has the issue or your friend? The way its worded confused me. I say if the chain/sprockets conditions are questionable, just replace them. $200 for new equip is cheap when compared to a hospital bill for broken bones or lacerations, not to mention that a broken chain can frag the primary drive case and tail section of the bike if its moving fast enough. The surging issue may not be chain related, and just be coincidence? Broken teeth would let the chain jump over worn or broken teeth, sort of like Brown57's thread pics - that sprocket was past done man! Alignment issues if you'd changed the rear tire and reinstalled it poorly. Usually when the rear wheel is in a bind or out of alignment, symptoms can be the rear brake dragging constantly and/or sprockets will have side wear on the teeth and the insides of the chains link plates will show wear or scoring.