I agree with FD. I would definitely tell him that it is an unapproved type and that you cannot sign it off. You wanna risk a confrontation with the CAA. Tell the guy to get real. He can't have it - and he should have gotten one that could be fitted.
One final thing G. Also tell him that if he wants to break the law as to CAA requirements you MUST report it to the authorities. That is what your authority says does it not?
How can he KNOW what 16 flashes my do. Has he had an EEG btw?