"...........the latter I think is scandelous, but there you are."
It might seem this way, but it isn't really; The fact is that debt collection is a high-volume low-margin business for those solicitors that do it. Banks and other clients with literally hundreds of debt collection cases are quite happy to pay a percentage of the recovery in debt collection cases because they recognise that this is what it takes to make it a viable business for the solicitor. It means that the good cases (where you actually get in the money quickly) subsidise the bad (where you chase for years, which is regrettably common), but that overall there is a viable business in the debt collection work.
Your friend might agree a two-tier approach with his solicitor: a flat fee for sending warning letters, and a percentage once it goes the route of full legal proceedings.
His solicitor might not want to offend by turning away the work: he should be firm with the solicitor and tell him\her not to take on the work if the firm do not habitually do debt collection work. I know I have in the past taken on debt collection work as a favour to a client, but the reality is that I served both myself and the client poorly by doing so.