I doubt he will be sued if he follow your advice because he is a foraigner
Exactly plus he does not even have a bank account in Poland and his home adress is in Norway,to sue someone over a flat you need to put some percentage of the total value of the flat with the Polish court and it is not worth it and takes forever.
I have a friend who was renting a place in Nowa Huta the tenants also had a year contract but they left saying they had found a place closer to work so she just looked for other tenanst ,that's all.
He is no longer happy with the flat, it is too cold and no longer suitable and just wants to go somewhere else , nothing illegal in that.