Check for fouling
Does your halyard have a steel thimble for the snap shackle, and the wear is on the narrow part of the thimble? If so I think your rigging only guy is dead wrong. This is based on my experience with the same problem on an O'day 28.If I had to guess, your spinnaker halyard runs through a block on a bail that extends straight forward from the front of the mast, and the spinnaker halyard straddles the forestay and the roller furler when not in use. The roller furler will hang on the spinnaker halyard and cause this wear.The reason both halyards are worn? Likely someone swapped the jib and spinnaker halyards at one time after noticing this wear for the first time. I've replaced both of mine and they're about the same length.The best solution for this is to find a way to eliminate the fouling.1. Change to a spinnaker bail that offsets port or starboard, so the stowed halyard does not straddle the forestay.2. When you are sailing, take the shackle end of the spinnaker halyard forward of the forestay, loop it around the front of the pulpit, hook it to the base of the forestay, then tighten the halyard. This gets it out of the way.3. Shorten the movement of the roller furler car. This was the solution for my boat. I had a 4" piece of cable between the top of the jib and the roller furler car. This allowed the car to come up far enough to get between the two sides of the spinnaker halyard. When I removed this cable, the car does not come up so far, and does not foul the spinnaker halyard. Problem solved.Hope this helps. If this is not the solution, I still don't think it's the sheave. They are aluminum and not likely to wear out, and since tou have a roller furler that sheave doesn't see much use anyway. Also, I think the type of wear you're seeing is more indicative of something moving across the rope, which is not how the sheave moves.Good Luck!Bob Amoss/v Simple Dreams