In some companies, the corporate rate is only authorized to be used for approved business travel. If so, you should assume that someone in the corporate travel office gets periodic reports of who used their rate, where, and when. Others might, when permitted by the contract, encourage employees and even retirees to use the rate code for personal travel also, including when traveling with friends and family (but not without the employee/retiree being present).

Any unauthorized use of a corporate discount could jeopardize your new job before you start. It could also create trouble for your friend when the company asks about the source of your knowing the code before the effective date of your new position. If you tell the hotel that you're a not yet new employee, they could contact the company for verification. All of this might be unlikely, but do you want to risk it?