How to get out unwanted guest staying with good tenant?

[Dave Charron]

One of my tenants in my 8 unit has been letting her son stay with her for the last several months.   He is not on the lease and I was not aware.  Apparently my tenant does not want him there any longer but has no way to make him leave so she contacted me.  In NY anyone that stays longer than 30 days has tenant rights.  Therefore I dont know how to get him out other than evicting both of them?

She pays on time and does not cause any trouble so, I would like her to stay but dont know if thats possible.

Any input would be appreciated.