Common law or divorced?

[harleybeepers]

My mom and my dad were married for 17 years and then after struggling with my father's drinking habits filed for divorce 5 years ago. The divorce is all said and done, my mom had got the marital home. (My dad signed a paper stating that as long as she didn't take his life insurance she got the house without paying half.) However, my brother and I are the only children who are now 18 and 21. When I had my daughter, almost two years ago, dad came back to mom's and pulled something about wanting to be back with his "family". They have been together since the birth of my daughter, but also since then things have been getting just as weird as when he was drinking. He also uninvitedly moved back in to mom's house, he is paying none of the bills and if anything creating more. (Trying to get a loan for a motorcycle of $15,000). He doesn't help her out around the house, he is basically freeloading on her, and then on top of it says that if she kicks him out he can take half of everything! She doesn't have that great of health to be under this kind of stress. I want to help her out so badly, but I don't know that much about Ontario's Laws.
So what I'm asking is, with this particular situation would he be able to take the house and half of what she owns and put her through hell once again? Or is he just trying to threaten her because he knows she doesn't know the laws very well either? PLease I really need some advice because I hate seeing her like this, and his threats just keep getting bigger.

If the home was awarded to her and the divorce is done he normally cannot change that property agreement--is that the case now?