Not to downplay the pot use, but what particularly is the son's concern?
Is your bf close to his sister?
How is the son going to explain snooping in his mom's room?
Yes, actually it does help. I would call the nephew back first and see what he wants his uncle to do. Maybe he just needed to vent, maybe he's nervous about the legalities of it.
I can see the mother getting upset if she thought her son was in her room, going thru her things, which he definitely should not be doing (his uncle might want to tell him that). I'm thinking, if he found a pot plant in the woods, that could be the excuse for his concern. Your bf could say that he was talking to the nephew and he mentioned that him (and his friends?) found a pot plant in the woods and does she know how it got there? Try and keep the nephew out of it as much as possible.
Of course she may deny knowledge, and it's possible it might not be hers, but your bf could take the stance "Even if it's not yours, you need to be concerned about who could plant something like that on your property, all of you could go to jail, because the police are not going to believe it doesn't belong to you. I'm not accusing you, what you do is your business, but kids aren't dumb, so if you are doing something, be careful not to expose your son to it, etc"
I welcome your thoughts, let me know if you want to talk more