Rat5. If one was genuinely self employed, one can choose which parcels of work to accept or decline. This is one of the core tests that HMRC apply in the UK. So, if your pal is "genuinely self employed" then he/she can just say ''no thanks'. But this isn't genuine self employment is it?
Having been through a HMRC inspection (different industry) it's not something to look forward to. I was playing according to the rule, but the inspection is so tedious and time consuming. HMRC have sweeping powers, you do not want to be on the wrong side of them. Your friend would be wise to get out of that arrangement, just storing up angst if HMRC decide to inspect.