Corporate codes are generally restricted for use by those who have some affiliation with the corporation, at least as a consultant or contractor if not an employee traveling on business. It seems to be asking a lot to expect someone to divulge their own corporate codes for abuse by a stranger. In the extreme, you could be required to pay the rack rate if you cannot supply appropriate credentials at check in to authenticate your use of such codes. In some companies, the person who gave you their code could be fired.