What would you do if you where me?

I am currently in a dorm with my roomate who is a very nice person and has been very helpful to me in money wise and etc.When I go see my parents out of town I usually let him use my 2009 Acura RL the only problem he likes driving barefoot which is no big deal however I have notice soil footprints on the carpet of my car how can I ask him to be careful without him getting offended any help is needed.