What Clapton and many other singer songwriters (indeed like myself) do, is use the techniques of the blues and gospel and American country music. That is to say a song like tears in heaven is permeated throughout with gospel and blues feeling. if you like, that is its traditional roots. the songwriting is what HE does with the blues. You don't have to write about traditional blues subjects - because you take the form as your starting point. You personalise it. But the blues is the basic building block.

In the same way that say Dave Webber or Peter Bellamy have used English folksong as a source of inspiration and technique.

Why you think its not valid if a person from iceland or Russia uses American roots music as a source of inspiration is your problem. the millions of people (like your friend) who can relate to this music tell another story.