assuming he doesn't already have racquet skills from other sports (squash and such) i think he should focus on:
- Being a good pusher: running down every shot and playing high %, low risk shots. (model after Ferrer).
- Do return of serve drills. Lots of them.
- Having a consistent good serve. Doesn't have to be the fastest or highest kicking, just very consistent and fast enough to start rallies at neutral or better.
It won't be pretty, but it will be effective in matchplay. If the guy has a squash background, you can get fancier and will want more net play and drop shots given he'll already have great hand-eye coordination, good movement when at net and should have pretty mean slices/volleys as well.
Disclaimer: I'm NOT a coach...
Real tennis: https://www.youtube.com/watch?v=yDqnkLJ9BtM