well Amber Lee's idea was good, then if she presses the matter further, just tell her, that you were not happy with the way she neglected their boxes. the cats need to be taken care of while you are gone, at least the cleaniness part, if not the attention too... they will miss you and then to have their box ignored is even worse for them.

Uabassoon, I would gladly do this for you, If I was not moving.
will your friend take better care of them than your roommate, and feed them?
I just hate to see pets neglected. when Muffin was my nephews cat, I went over there every day while they were on vacation and cleaned the box, fed her and gave fresh water, and let her out of the laundry room for about 3 hours to run and play.

I really wish I could help with the kitty cats.