If your friend knows how to tighten the valve stem on the faucet, you might try that first.
If the valve cartridge is shot, it will have to be replaced. That requires shutting off the water to the shower -- or maybe the whole house -- and then removing/replacing the cartridge. This is not hard if you have the skills. Go to a Home Depot and get help.
Last resort, call a local handyman (see Craigslist) and have somebody do it while you watch. You can learn a lot and save money next time.