I'm with mooman. Seiko doesn't make my favorite tuner anymore. It's big, it's clunky, and I've been known to leave it behind sometime. But tuning forks are the best. I don't particularly like those new fangled tuners because they are hard to see for those of us musicians with low vision. Luckily I have good ears, and deafness, so my MD uncle assures me, does not run in the family. Perhaps your friend is as lucky as I. Many blind or near blind folks do have excellent hearing.
    As far as tuning my fiddle, I use an A-440 tuning fork, and listen for the beats. These are the sound waves, I guess that are doing their thing. When the beats go away, I know the instrument is in tune. Putting the fiddle up next to my collar bone in near playing position, I can actually feel the wood vibrating in a peculiar manner if any of the two ajacent strings are not in tune with each other. Feeling the wood vibrate in this manner can also be done on either guitar or mandolin, too.
Jim