My partner has had a separation with his de facto ex-girlfriend for almost 2 years, and has a 2.5 year-old son with her. We have his little boy for 3 nights every second weekend and that is all at this stage. She pays her child support, pays for her brand new car, set up an apartment for her and has spent upwards of 30k over the past year paying for whatever she has asked him for. He has been totally kind and agreeable the whole time. But she is mentally unstable, and his son is much more relaxed and happy in our home. We both work and provide a stable home for him, and we are about to have a baby. We want primary care of his son so that he can grow up with his sister and have the life he deserves. His ex withholds him all the time for no reason and makes life very hard for us despite my partner doing everything he can to provide for her. We are about to seek legal advice and he is currently going through mediation. I am just looking for some advice or peace of mind about our chances of winning this horrible battle?