on behalf of a friend......he sold an item to someone, a consmable. After they had used up 26 of its 30 consumable parts, they rang my friend to say it was half used but now broken  Brian being a decent chap said no problem return it & i will replace with another one, however on receipt of it, the item as mentioned above, had had 26 out of 30 of its units used up (not half atall!) and the remaining 4 were fine, not broken atall  so whats the answer? return another one with just 4 remaining units just to be pedantic? Is this legal? As the unit is meant to be sold sterile this might not be feasible so i reckon divide unit value by 30, multiply by 4 & thats their refund!!! legal or not? what does everyone think please?

They mislead him and I can't see they are entitled to any sort of a refund after happilly used so much of it.  I would return the "thing" with the 4 units, perhaps taking a photograph (helped by a decent photographer perhaps ;)) of their unbroken condition and let them try and take action.  Fail in that, I would follow your latter suggestion but again taking some photos for evidence if they try to take anything further.

If he has checked it and the remaining units are unbroken, then they are not entitled to anything. I would decide how much the customer's goodwill is worth and the likely value of repeat sales, then decide how generous to be. Why not sell them a new item at full price less the value of the  4 units that they claim are unuseable. At least you get a repeat sale

If I were your friend, I would check with 'Trading Standards' as to how he is legally situated.  I am assuming that he has a business.  However, I think that private individuals selling items don't have the same legal requirements.  I am sure that someone else will come along who knows the answer to that one.  :)

I would say send it back but the wording say it was half used but now broken  is confusing me a bit Sam. Sorry.  That suggests one item or a requirement of others to make it work.. if that makes sense. Can you not say what the product is so it makes sense?.

If the 30 parts all function individually and do rely on each other then I would tell him to jump. I don't think anyone can actually say it is legal or not unless you can understand the product and the reason for it to be 30.. Sorry to be ackward but legal generalising answers sometimes causes more confusing

That is why I suggested that the person check with 'Trading Standards'.  Businesses do have legal obligations to their customers regarding the supply of goods. (Sale of goods Act!)