I have found it is often so called friends who think they know better and go off and buy elsewhere and get it wrong.They then probably expect you to help them out for nowt.....Firstly,does the supplying dealer still operate.
Easiest resolution is to try and ask the dealer to refund them less a small amount for usage. The kind of dealer that hides write off damage will usually just say **** **F!
Second easiest (assuming you can not sell a Cat D car) is for them to try and sell the BMW privately for close to what they paid for it. Luckily because the car is so old but not quite a classic the difference in values may not be massive and there is always someone on the market after an e46 Convertible.
The next option of course is for them to start legal proceedings against that dealer to recover the price they paid for the car.