Tell him to get a .444marlin, .450Marlin, or .45-70...or maybe a .38-55 or one of the .35's you mentioned. A LOT of KO power in these rounds.

Any cartridge can make a deer fall, and as you pointed out, even on great hits with high powered cartridges, sometimes deer run off anyway. Throw a bigger bullet at them and you've got a better chance of stopping them flat out, energy or not, the taylor factor of big bullets is more likely to produce dead falls.