A loan from a friend is a personal matter, and I cannot see how the house is being bought into it.
Secondly, if your sister has children, then she cannot be forced to seel the house until the children are ( I think) 16. And certainly not by her ex husbands friend.
Thirdly, To my very limited knowledge, a decree nisi is part of divorce proceedings, maybe you mean he had a charge placed against the property, highly unlikely in a personal dispute matter.
I strongly suggest your sister seeks proper legal advice.