Marriott properties require documentation from the hotel (where your roommate works) before they will allow someone to use an employee rate or friends and family rate. If you show up without the right documents it's likely they will require you to pay a rack rate.

My DIL worked for a Marriott property a few years ago and there are definite steps to follow before friends and family can get a discount. Just using her employee discount card won't get you the Marriott employee rate.