If the plants are at someone elses house who doesn't have a MMC wouldn't be protected. Here's a link to NORML legal issues:
Legal Issues - NORML
Basically cops can say anything to get you to tell them something that they can't force you to. Why didn't he take you to jail without the prescription? Because he knows he can't.
I'm glad to hear they didn't hold you friend in jail over 2 plants. Sorry you lost your plants,too.