As commented by @Daniel R Hicks the problems I would be concerned about are to his own health. The bike components that will wear out prematurely are simpler to replace.

Potential health issues are knee problems from "mushing" all the time. MTBer's tend to run lower cadence than road riders, for valid reasons, but when on the road tend to run closer to road cadence. Mushing causes excessive pressure on the knee, causing the joint to wear out - its an Orthopedic surgeons retirement plan, in 20 years time he will be doing knee replacements.

The other concern is mixing it with traffic in high gear. You have limited ability to accelerate and your balance is often compromised. Under usual conditions, not a problem, but in an emergency that could be the difference between a close call and a grease mark on the road.

Less of a concern, and people physical attributes vary, is endurance and aerobatic capacity. The reason for spinning faster is it provides maximum aerobic efficiency. Under about 80RPM you are not able to maximize the power output of you legs. Beginners often spin too slow, as the optimal cadence needs technique and practice to learn how. To some it feels you are going faster mushing as it takes less effort if you spin faster......

As commented by @Daniel R Hicks the problems I would be concerned about are to his own health. The bike components that will wear out prematurely are simpler to replace.

Potential health issues are knee problems from "mushing" all the time. MTBer's tend to run lower cadence than road riders, for valid reasons, but when on the road tend to run closer to road cadence. Mushing causes excessive pressure on the knee, causing the joint to wear out - its an Orthopedic surgeons retirement plan, in 20 years time he will be doing knee replacements.

The other concern is mixing it with traffic in high gear. You have limited ability to accelerate and your balance is often compromised. Under usual conditions, not a problem, but in an emergency that could be the difference between a close call and a grease mark on the road.

Less of a concern, and people physical attributes vary, is endurance and aerobic capacity. The reason for spinning faster is it provides maximum aerobic efficiency. Under about 80RPM you are not able to maximize the power output of you legs. Beginners often spin too slow, as the optimal cadence needs technique and practice to learn how. To some it feels you are going faster mushing as it feels like it takes less effort if you spin faster. I find a good test is to get them to change down a gear, keep the same effort and monitor their speed. Most are surprised to see an instant increase of 1 or 2km/h for no more effort.

If you are really keen to get him to spin faster, put him on a bike with short - really short (like 145mm off a kids bike)- cranks for a few rides. As you have much lower torque, you have to spin to go fast, but spinning smoothly is easier, so you just do it. It's a great training aid.