Extra person in rental, utility bills

[Michael Schnyder]

I have a 3/2 rental that my daughter lives in with two other females since September.  The lease agreement states that the three
of them live there and if anyone else should live there on a regular basis, they would be in violation.  Around October, one of the females
got a boyfriend that is now at the house almost every night.  The problem is that not only is the rent way below market, but I also pay all of the utilities (both to help my daughter out).  The agreement was that I would pay the utilities bills provided that they don't drastically increase over time.  Now that the boyfriend is staying there the one tenant is violating the lease, not only that, but the utilities have gone up with the boyfriend now there.  I've already texted the female tenant last month saying that because I pay the utilities he couldn't stay there on a regular basis or she would need to pay an increase.  She simply replied that she understood, however according to my daughter he's been there almost every night since.

I wanted to get some advice / help on what to do.  Do I ask her for a percentage increase when paying the rent in January, if so, how much of an increase can I ask for?  Do I confront her sooner to let her know that I'm aware that her boyfriend has been staying there and that she's in violation?  Based on what I've said, I believe she's in violation, but maybe I'm incorrect about this?  This is my first rental property so I'm still learning as it goes.

Thanks in advance.