First Problem with my tenants

[Andrew Hughes]

I have had my first property, which is a 4 unit, for 5 months. Everything has been perfect so far. My tenants are two guys that have been there for over 30 years, an elderly lady, and a mid 20s male. All four have been great, paid rent early, and cause no problems.

Today the lady sent me a text saying she smelled marijuana in the young man's apartment and wants to leave because of this. They have lived next to each other for ten months and she said this is the first time it has been an issue. She asked to be let out of her lease early, which I said no to. I told her I would discuss the issue with the other tenant.  

My questions are first how binding is the lease? She didn't make an issue out of it, but if she just left early what would my recourse be? Also I don't care at all about the other tenant smoking marijuana as long as it doesn't bother the other tenants. He is a quiet guy that keeps to himself so other than this issue he is a model tenant. How should I go about addressing this issue with him? Any feedback would be greatly appreciated.