Shakeel, this is actually an area I know quite a lot about. In a previous existence I was a debt collector.
The advice to your friend is very straightforward. He should tell the debt collector that, having taken advice, he understands that the Spanish civil debt is not enforcable under English law. That, should the creditor wish to proceed with the matter, they should proceed through the proper legal channels.
He should further state that he will not deal with the matter any further other than in writing and that he will regard any further phone calls as harassment and act accordingly.
He should be polite but very firm. Collectors will often say anything, often downright lies, in order to collect a debt (and their commission).