The old fashioned way of measuring was to stand next to the ski and the top should reach the tip of your nose. So,
assuming he is a normal person - that would be approx. 8" shorter than his full hight. (5'8")
Risky business buying someone ski's as a present, gift vouchers might be a better idea. Ski lenght used to be done on the ability of the person. I'm 1.73cm and used to ski on 195's but since the introduction of Carving Ski's lenght in not so important.
Thats what I've told the wife anyway.
Voucher seems a better idea - there's a lot more into buying skis than length. Weight, style, experience, off piste/on piste all make a difference. And the red ones tend to go faster too.