You must provide specific reasons for denial.
This is NOT a privacy issue. Page 13151 of the Federal Register, dated 3/18/03 makes it very clear:
Many commenters were concerned about the co-applicant’s or guarantor’s privacy when the reasons for adverse action pertaining to creditworthiness are given to the primary applicant. When a person agrees to be a co-applicant,
guarantor, or similar party, however, there is (or should be) a general understanding that information will be shared. Accordingly, the rule has been adopted as proposed.