They had a joint account in Spain and a credit card each
So the bank account was in joint names, were the credit cards on a joint account or were they separate? If the account was in her husband's name and she was an additional cardholder, then in the UK he would have been the liable party and (if there is no estate) the debt dies with the account holder, unless the additional cardholder has made any purchases since the death. If it was a joint credit card account then both are equally liable. Credit cards are a form of unsecured debt which means that the debt cannot be placed against the property or someone else being made liable for payment. Of course, the bank may issue court proceedings if the amount is worth their while and they rely on the plaintiff not turning up at court, as they win by default. I have been to some of these hearings in the past (only in the UK) and the large organisations on't even send a representative which means if you turn up, you win!
I can't see there being any difference in Spain as the credit card industry has had to come up to European Spec in the UK which should mean Spain has too (although we know the banks have their own rules!!).
Best bet really is to tell her to seek some legal advice. It will probably cost, but it may save her money in the long run by not having to pay debts that aren't hers.