Not sure what to think. My first response is that Polish families don't behave like this unless there's something else (fairly serious) going on.
She was under alot of pressure from her family particularly because they had been building a house together and she had invested alot of money
Aha! If this isn't an elaborate smokescreen, then my best guess is there are money issues that you don't know about (she might not know about them either). If I had to make a wild guess it would be that the parents are in debt and/or don't want to lose the money/work sunk into the house. Also, what part of Poland is this? Another wild guess might be that he's a smalltime criminal (or from that kind of family).
In a different direction, I basically know of only a few other kinds of situations where the family will side with the boyfriend against the daughter.
1. they have a kid together that you know nothing about (premarital sex is the norm in Poland, birth control isn't, I wouldn't even rule out the 'boyfriend' being her husband and she was hoping to be able to negotiate a divorce when she went there)
2. she has a history of mental illness (high stigma that never wears off)
3. severe family dysfunction (including physical/emotional abuse)
4. the foreign boyfriend is not white (no delicate way to put it)