After they split she initially said that she did not want anything from him, however, she is now demanding that he should pay half the loan repayments, and has threatened him with legal action.
Normally, (in the absence of child support claims) the financial split under Polish law is 50/50 with some exceptions (personal effects, items necessary to ear a living, pre- and post- marriage property, inheritance, copyrights etc)
For this reason she cannot keep the entire car but pay only half of the cost (that would make the split 25/75 to her benefit). Moreover, Polish courts may consider the degree in which the man and the woman contributed to the financial situation before they were legally separated as well as blatantly dishonest financial moves on the part of one of the partners. So while there are some general rules, the court will consider individual cases form what it would consider a fair approach.
However, I am not a lawyer and this forum is unlikely to be a source of reliable information on the subject.
(in Polish, but perhaps someone could help there)