Insulation in the joists above is a must. Also, be very very very careful cutting any holes for can lights, etc. as you're defeating the isolation unless you box them.
If he wants to take the next step, build the walls 1/2" short and use PAC DC-04 clips to decouple the walls from the structure to help with bass transmission. Add hat channel or RSIC-1 and hat channel for the ceiling.
Pay attention to HVAC as that is a direct path to the rest of the house no matter what else you do.
One more level - add a 2nd layer of drywall with Green Glue between layers. This helps significantly in the low frequency transmission which is the hardest to deal with.
Bryan