Depending on the amount then you should see a lawyer. Yes in theory you would be liable for the interest and if the claim is made within 10 years then you are at risk. However it is arguable that you can only be liable for interest that the dead man agreed to - if for example they increased the interest rate after he died when he would obviously have not been able to answer letters then you can argue back against that.
If your wife reads Polish then maybe she can look at the website antywindyk.pl - there is plenty of advice about how to deal with ambulance chasers and debt collectors.
