My diabetic obese friend has foot pain and some numbness. Is it ok for him to use a menthol gel like biofreeze (2-4%) menthol, on his feet?

Control risk factors. Menthol is ok but it is only treating the symptom. Your friend needs to treat the cause of the foot pain/numbness which in a diabetic is a signal for neuropathy. He needs to control his blood sugar level, blood pressure (circulation) and cholesterol levels. Good luck.
It may help. It may be of benefit in some cases.
Maybe. If the skin is closed it should be ok. The problem is its unlikely to help.
Yes. Biofreeze is ok to use but your friend should probably esamined by a podiatrist to see about the pain & numbness.