My new neighbor loves to smoke in his bathroom with the bath fan on. Not a big deal, but we live in a condo and apparently the developer cut corners, because his cigarette smoke smell fills my house every time he lights up.
We've narrowed down the issue to the bath fan- it looks like we share an exhaust with this neighbor.

I just remodeled my bathroom last year, so I don't want to do any tearing down of walls... Is there anything I can do to block his smoke from entering my house?
I was thinking of some sort of filter.
Turning on my bath fan tends to clear the air, but I'd rather not have to leave my fan on 24/7.