As far as your rights are concerned, by working in a bike shop and agreeing to work on your freewheel without any warning that it may go wrong, the mechanic, if not the shop will owe a duty of care to your property. im fairly sure you are entitled to have the situtation put right. In this case a similar freewheel in similar condidtion to yours, let me know what they say and i'll give you some cases to throw at them