Help with friends subleasing

[Brock Hoffman]

I have a question for you all where I would like some feedback. Friends and I are renting a house for my last year of college, and we decided to sublease to two of our other friends. They paid all the rent for their time there, and one of our friends decided to pay for all the utility bills and then have the subleases pay him back at the end of the summer. It just made things easier. They are very trustworthy friends and we had no doubt they wouldn't pay him back. Well, they just moved out (not quite the end of the summer) and my friend wants them to pay him for the utility costs. What is the right approach to go about this? Should you just ask if they can send a check by a certain date and then move on with it, or should you send a very strict and "professional" email? The email that he sent is below:

There is no contract that can make them pay the "fee of $10"

This was just a friend to friend sublease that has gotten slightly out of control.

I appreciate the feedback. This will be good for the future when I actually have tenants to deal with ;)