I just dont think its fair to tell somebody that they have to wear a particular thing (at your request!) and then expect them to pay for it! Wedding etiquette aside, its just damn rude!
I think the answre for this is for the best man to get back to the groom and say
'I'm sorry, but I cannot afford to pay £50 for the suit - can we talk about alternative options?".
If the groom offers to pay in full, alls good.
If the groom says "Well I cant afford to pay for it all" then the Best mans response is "Ok then - lets rethink the suit thing so that it works for both our budgets" and suggests any of the following situations which he is happy with:
-Wearing a suit he already owns
-Hiring a cheaper suit which best man can afford/justify/is prepared to pay for half of
-Buying a suit and paying for half each, which best man gets to keep (best man may feel this is a better deal for him - he still has to pay for half a suit he doesn't really need - but at least he has a suit to wear again or sell).