My daughter is in college and has a roommate that has been trying to sub-lease because she is going to GA for a school internship. My daughter asked the landlord if the locks could be changed and notifiy the roommate that when she returns she has the newe key at the office. The reason for this is because my daughter says she does not know who has keys and some of her things have come up missing. The landlord does not want to change the locks. I'm not sure how to approach this issue with the landlord

You approach it the same why you did here, but offer to pay for the lock change if your daughter is missing items and he is not willing to do it.