Yes, this is very worrying but I suppose the advice would be the same as here in the UK. Do not stop if you believe the car in question is NOT a Police car. Do not gesticulate or do anything that could further incite them. Try to get to the nearest garage or busy place if you need to pull over. Always report such incidents to the Police as soon as you can. If you are caught out and they have guns, remember goods can be replaced but not lives, it isn't worth fighting back.
Car jackers will roam roads close to airports where they target tourists straight off the plane in hire cars, or possibly motorways across the border where they target those with foreign plates, of course British cars stand out a mile as wheel is on the right!

Try not to carry large amounts of cash, you can still get travellers cheques or top up cards to carry cash.

I would suggest that the family who were robbed contact the British Embassy in the UK to report what has happened and ask that they speak to the Embassy in Barcelona ro reley concerns about tourists being targetted in this way.

Spain need to take responsibility for this and do more to prevent this sort of crime.