I would suggest you print up some care sheets with lists of diets and heating and lighting needs. Rather than make it seem your going after him try the approach hey since I was caring for your turtle I decided to do some research to give him the best care I can and I thought you may enjoy the tips I found. Thus saving your friendship with your roommate and possible embarrasment.
Also as a box turtle owner myself Id suggest getting rid of the carpet, it gets dirty quickly then bacteria just sits in it. I use aspen shavings, its cheap and easy to work with. We also provide our turtle with a hide (a coffee container with a hole cut out) and she loves it. And it should have a more varied diet to fresh fruits and veggies (no lettuce). Our turtle loves fishing worms, meal worms and crickets. But will only eat red fruits, she loves watermelon nectarines and strawberries. And uvb lighting needs a fresh bulb once a year I was told.
Hope any of this helps you.