The first step is to urge your friend to talk to her ex about this. After all if her plan is for him to take him then he needs to know that she plans to leave the country, no?
You need to voice your concerns to her and be very firm that you think she is making a big mistake, she will do what she pleases with that information but you have to speak your truth.
Then once the cat is out of the bag and everybody knows the plan, you will be able to see where the cards may fall. This "plan" of hers could completely backfire anyways. Don't panic until you know it's actually happening.
Also, if you're really close to the kid make sure you express that to her and the dad. The more a child is loved the better. And if he the dad knows how truly close you are then it would make it harder for him to up and leave the area.