I once thought, many years ago, that we had a marijuana plant growing in the back yard (ex was sober many years in AA, so he wasn't about to be growing dope). I took the leaves into my office (law-enforcement type), and nobody could figure out whether it was or wasn't.
In the end, it turned out to be a variety of Japanese maple (seedling), but darned if those didn't look like mj leaves.
A few months later, there was a story in the paper about some guy who had people sawing off actual BRANCHES of a BIG version of this tree, in the apparent belief it was what its leaves looked like (though how anyone can think mj grows to be ten feet tall is beyond me).
Sorry for the digression. I agree with anvil--it ain't good, but it isn't like having a meth lab in your kitchen. OR a serious "grow" operation. You can insist he get rid of them, but what's to keep him from repotting the pot? And just moving it into a sunny window?