Depends on the tear. Usually, the meniscus only affects your mobility if there is partial cartilage sticking out. Surgery should fix that given they trim that off.
Since cartilage can't grow back, rehab is the most important thing for him right now. He will need to strengthen his knee (quads, glutes, hamstrings, etc) to avoid adding pressure to his knee. He does all that, he should be fine.
It's amazing really. Torn cartilage is like having a 6th sense almost. My knee always aches when the weather is about to turn cold or if its going to rain.