In answer to your question, Yes, they should declare it on their Income Tax return. If they don't declare it and someone "shops" them it may end up costing them more than just the unpaid tax. What happens if the person your friend did the work for claims the £1000 as a business expense and lists your friend as the person who they paid the £1000 to?
Anyone who works for cash-in-hand without paying tax is committing tax evasion.
Apart from that, the more these people pay in tax the less the rest of us has to.