sister in low want the house

Need some help here...


My friends A is from Australia, he has a house in Australia, still paying mortgage. He travals a lot for work, so not home alot indeed. About 9 years ago, his brother died, and his sister in law didn't want to stay in her own house anymore. Since his house is big enough and empty most time, he offered her&her three kids live in his house. Now 9 years later, he found a serious gf, and he wants to settle down with the gf, he wants to move to a new place and sell the house he has now. And his sister in law wants to buy it. He is willing to sell the house to his sister in law 10% less than the market value, but the sister in law want it to be 50% less than the market value. He is not rich and he still owes the bank money. He refused his sister in law's suggest. Then something amazing happened! The sister in law got a lawer, and she start telling that he and she had a relationship, and she uses the fact that he never charged her the rent as a proof, trying to get half of the house! So my friend A now need to know what should him do to aviod lost! Thanks!!!

well, what to do? get a lawyer of course

Already did that...just thought maybe can get some other useful infomations here.