How can I stop her from going if the court approved the move?

Three years ago, my son and girlfriend had a baby. They were both into heroin at the time and signed custody over to her mother. Since that time, both mother and father are in prison. I keep a close eye on the boy as her mother has been in prison in the past and in trouble with the law. Here about 4 weeks ago, she got kicked out of her house because her boyfriend was cooking meth. I talked to her case worker and they continue to monitor the situation. She just informed me she was moving with the kid.