Are you being serious here?
If I find a cornish pasty doesn't mean I have to take it back to cornwall
Meh sorry I'm just messing with you!
I would have said that Scotland isn't special and uses the exact same currency as the rest of the UK.
But in this case this popped into my mind:
http://en.wikipedia.org/wiki/File:Bank_of_Scotland_20_pound_note_2007.jpg
In which case your friend is right in saying it's useless as I would have thought that it wouldn't be classed as legal tender.
But there's no need to bin it at all that's silly, I would hang on to it as a collection.
After all they aren't very common, and could be classed as a collectors item.
In fact if it is that same note I mentioned above then you could if you find the right person sell it for anywhere between 30 to 40 GBP.
I know these things pretty much as I collect coins and notes myself. (Got them from all over the world now).
Let me know what note it is exactly though and then I can be absolutely positive.
Kind Regards,
Matthew