Possible Lease violation but unsure

[Dan Huntzinger]

I currently have a tenant in a year lease agreement, changing to a monthly lease on June 1. One section states that "the number of occupants shall be occupied by no more than 3 persons, 1 adult and 2 children under the age of 18, without written consent of the Lessor. "

She has a new boyfriend, who is not on the lease, who spends an average of 10-12 days a month staying there and is not on the lease. I do not unfortunately have any clause that specifically states Limits on Use and Occupancy (this is my first rental and I need to rewrite a better lease.) 

My question is, is this a violation of the lease or something I can't do anything about as I have no specified number of days on my lease?

Thank you for all the help