First check brake fluid. If there is none then refill resevior and bleed brakes. Look for leaks.
If there are no leaks in the system, check for leaks from the hydroboost. If no leaks are found from the hydroboost. Then three things could be the problem.
Either the powersteering pump is failing or the Hydroboost is failing, or it is the brake pedal assembly.
Check the level of the powersteering pump fluid. Does he have good powersteering. If he has good powersteering and the fluid level is correct. Then the only other two things it could be is the Brake Pedal Assembly or the Hydro-Boost itself.