Graphite may not actually be a poor filler. In fact, it might be really good. Experimentation reveals all, though!

Have him try a multiple experiment with some blocks of edge-grain wood (3.5" x 3.5" mating surfaces will be sufficient):

1. One pair of blocks with plain epoxy - no clamping force.
2. One pair of blocks with plain epoxy - heavy, uniform clamping force.
3. One pair of blocks with graphite-filled epoxy - no clamping force.
4. One pair of blocks with graphite-filled epoxy - heavy, uniform clamping force.

See what it takes to knock them apart with a blow that's parallel to the seam. The differences should be quite appreciable.