Really need advice

So my brothers fiance up and decided to leave him and take their 2 kids and originally they just made a verbal agreement that he would have them every other week and he would buy them diapers and clothes anything they needed and he would buy her a car and continue to pay her phone bill so that he could talk to the kids everyday and so she could get them around because she has never worked and he has always paid for everything well she decided she wanted to move to another state he said no if she does that he will take her to court so now she's saying she will take him to court and take full custody of the kids and not let him see them so my question is what are his chances of getting custody especially regarding the oldest child I don't think he will have any legal standing with her as she's not biologically his though he has raised her and provided for her, her whole life. So what should he expect? What can he do to better his chances of winning or at least getting joint custody if anything? I'm tired of watching my big brother cry and stress over whether he will get to see his kids and whether they will be taken care of because she still has no plans to get a job she plans to live off government assistance and the child support he will provide. Any advice/ tips will be greatly appreciated.