My neighbour has just brought her cocker spaniel over because she seen hairs on her tongue, sure enough she has several long black hairs, we've plucked a couple out and googled and apparently this is a thing, can't find how to cure it though?

Reading up (as never heard of it before) according to this vet dentist site If it's actually hair unless theres infection in the skin (in which case seems the skin in the grove can be removed) they say to leave them as they will just grow back. I can't see any mention of causes for this. your friend could try contacting them for more info and advice.
http://www.vetdentsa.co.za/Hair-On-Dogs-Tongue.html

Id get a tooth brush and give the tongue a quick scrub to keep it as clean as possible after meals.

There is a separate condition called hairy toung that can also occurs in humans too but this isn't actual hair but can apparently look like it, can be caused be some meds, lifestyles, infections. Treatment can include tongue scraping/cleaning, change lifestyle, antifungal or antibiotics if needed. Unlike the hetrotopoc hair this one doesn't just grow along the grove but spreads out over an area.

Had to google this as I had never heard of this before and came across this series of posts on a cocker spaniel forum
http://www.cockersonline.co.uk/discuss/index.php?topic=39272.0

Thanks both, I was wondering if anyone had experience of it, but it's not bothering her so I guess it's fine. Her teeth are great, she gets lots of raw meaty bones so no real oral hygiene issue.

I wonder if, with day to day use (eat, licking, playing with toys) it'll be worn away? Be interesting to know so keep us posted