Out of interest what set up did he use - long waggler rod? open faced reel? trotting of the rod end or mid river? etc.
18mm is a big pellet and hence must = big hook. Did he not have any problems foul hooking fish?
A friend of mine has used a similar set up in a fast shallow swim on banded pellets but continually foul hooked fish due to the exposed hook. He didn't have the same problem with baits directly on the hook.