Depending on the layout of the yard, the police can't just go in and do something without a warrant.
I don't know the law where you live, so it may not be the drama you think it will be. Some places, he would probably get a summons for municipal court and pay a fine or get a diversion.
Hope it works out the way you hope it will. I suspect, though, even if he stops growing the plants, he will still have marijuana in his home.
Good luck,