Law /
Last will in Poland - (can a stranger inherit everything?) [40]
These are serious amounts of money - all the advice is correct above - especially to get a lawyer in Poland.
A few other considerations !
The Polish police have got better things to do than arrest people trying to attend their fathers funeral. You can only be arrested for committing a suspected crime. If it is in church she is free to turn up.
If this lady is not a US citizen and is not married to your FIL then she could face an inheritance tax bill in the US of several million dollars - it depends on on many factors. Unless she has liquid funds to pay that then this possibility could give you a huge advantage in getting her to be more rational. Speak to a lawyer in the US who specialises in inheritance.
One question your partner will face is why she did not speak to her dad in the 9 months after he lost his wife. If that was me I would be checking my dad every day to make sure he was coping, so there is obviously some backstory here that we are not told. and it's possible that this woman was in the background for some time - she has every right to the 50 percent portion but not to the portion reserved for relatives.