Rev Leon Lonnie Love
damned mine eyes, DAMNED mine eyes!!
To combat hair loss, you need to attack on multiple fronts, but the most important for the present and future is to be preventive.
Your "friend" will need a DHT inhibitor. Finasteride will do. He can get a prescription online for easy via Keeps or Hims, or he can go to a local dermatologist.
Minoxidil (brand name Rogaine) will help stimulate hair growth. It works better in the crown but it can help thicken up what he does have by encouraging growth.
Lastly, a hair transplant, which can supplement your existing hair and/or replace that which you've lost. This is the most expensive option but by far will make the biggest cosmetic difference. This should be the option he takes further down the line, however. Stabilize & prevent/slow first with minoxidil and a DHT inhibitor. Less hair to transplant = money saved. Hair loss will also continue after a transplant if you aren't protecting the native hair that's susceptible to miniaturization due to DHT.
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And i say this as someone who has done everything except a hair transplant.