Zone 1 or 2 work (cardio you could literally do all day). Instead, of spending much time in the middle zones you will get the biggest bang for your exercise time effort by doing high-intensity interval training (beyond your FTP). Is his cycling coach ensuring he gets plenty of interval time? It really stimulates the improvement in strength/power.
Strength training like squats, weighted lunges, planks, etc really helps too.

Hopefully, he treats his rest/recovery as seriously as he treats his diet and exercise. Exercise/stimulus is one part of the equation but rest/recovery is also required for peak adaptations. With all of the 5k walks he's been doing, in addition to the bike/trainer work, really demand recovery. Hopefully, he isn't neglecting good sleep.