Got his request from someone, thought some discussion would help him out:

My disc 4 of the BBC LOTR set is damaged and won't play the last track. Is it possible to purchase just the one CD somewhere -- my set is in good shape and it isn't worth replacing the whole thing for one track! Thanks -- I really enjoy this production. Ian Holm is great.

Some ideas:
  • The publisher might have a disc replacement policy
  • eBay
  • a fellow collector could help out