I would decline the candidate's offer of his employers code AND I would further notify the candidate that he is no longer under consideration for employment with the firm.

Whatever code from his employer he is offering for inspection, rest assured that at some point in the future when he'll be looking for another job, he will do to you what he is doing to his current employer. 

Of course, the picture gets worse for your candidate if your candidate signed on an NDA with his current employer because now, he is in possible violation of that NDA. Again, rest assured that if he is willing to violate his current employer's NDA, there is nothing to stop him for doing the same to you when at some time in the future, he is interviewing with his next prospective employer.

If you need to evaluate your candidates' programming capabilities, review their Githubs and/or give that candidate a take-home project of your devising. Then have the candidate review their code with you.