Marijuana smell in my rental

[Megan Fisher]

Hi Bigger Pockets Community - I am a first time forum poster.  I've been enjoying the podcasts for awhile now and have learned quite a bit hearing all your stories.  This made me want to bring a question to you all.  Today, my tenant called to let me know that she is smelling marijuana in the bathroom of the condo.  She thinks it is coming in through the fan in the ceiling above the shower. The bathroom wall is the only shared wall with others.  This could be true based on the fact that the fan vents into an attic shared by other units.  Rightfully, she is concerned about this due to children living in the home.  I know that pot is legal in CO and homeowners have the right to smoke in their place, so I don't see a conversation with the neighbors being fruitful or appropriate.  My question for this community is: what mitigation strategies do you know of to prevent smell coming into the unit?  Specifically, do you have ideas utilizing the fan/vent above the shower?  Thank you!