They will die.
Marijuana is one of the more difficult plantes to grow because of it's precise growing and light cycle. If he doesn't have proper humidity levels, oxygen vs. nitrogen content, and precise lighting, they won't make it past a few weeks.
If your friend wants to smoke this "stuff" then his best bet is to leave them where they are until they bud. And that all depends on what type of plant it is. Although a hemp plant produces THC it's quantity is not greater then the THC in a head of Kale lettuce.
But remember this. Cutivation of Marijuana is a felony offence in every state in the United States within some limitations (
Find laws in your state). If your friend is caught with these plants he will most likely go to jail.
The "legal" thing to do would be to call the police and show them the plants. They will take care of them and most likely not hold him as a suspect because he informed authorities of their existance.
If they are on his property (Rental or purchased) and someone else turns them in he will most likely be heald as the primary suspect if not arrested to the cultivation especially if he's a "stoner". And, If they find any marijuana on him, in him, or in his house the odds are that he will be convicted.
-Al