If he is shooting them with a 20 gauge slug and not finding them it is no fault of the slug. Any deer I have seen shot with any style of slug from a 12 or 20 gauge was dead quick, fast and in a hurry.
Sounds like he needs more trigger time or buck fever is getting to him and he is not making good killing shots.
As far as slugs go I had my best accuracy in an Encore 20 gauge with Remington Copper Solids, I believe these may be discontinued but not for sure.
What I learned with slugs was to try 3 or 4 different brands and styles. When I did this there were clear differences in accuracy. Like I said the Copper Solids were best shooting 1 1/2 groups at 100 and the worst were Hornady SST shooting an 8-10" group with the others I tried falling somewhere in between.
