YOU can't do anything. except write up a unauthorized tenant living in unit as lease infraction. that will either be remedied by adding the new person on or booting everyone in the unit.The police won't force the new roommate out.Check landlord tenant laws for your state and comply with that.As long as current tenant gave access to the unit, it's the tenants problem not yours unless you press the unauthorized tenant issue.. as far as my experiences have shown. 

Why that particular infraction. 

Could be a serious infraction. 

Then I told him who I was and how many infractions of legal issues and ethical matters he was tramping all over.

Here is my Short Package if you care to read an Affidavit http://www.yellowlettermail.com/Documents_Needed.pdf...and technically, driving 56mph in a 55 zone is a punishable infraction as is spitting on the sidewalk, but I'll take my chances.

Others will say there is value in fractionals etc. but it's just not my cup of tea.

There seems to be a never ending stream of License Fees, fines for tenant infractions (not cutting lawns, outside furniture on porches, etc) but at the end of the day.... a cleaner, safer neighborhood is in my best interest.  

There has been some rumblings of people getting denied STR licenses for some code/zoning infractions posted on another BP forum post.  

Any infraction can be challenged by a hearing.

In my area, city inspectors will write you up for an infraction for windows that don't open, only open halfway, or are very difficult to open.