The judge wouldn't make a ruling simply after hearing grandmother's side; s/he would generally agree that all parties need to go into mediation.
Frankly, grandma has a problem - a huge problem - and mediation actually might be her best bet.
Grandparent custody cases are very, very difficult to win and usually they require that the grandparent has had the child/ren in their care for years (or at least a very good percentage of the children's lives). Five months is not generally considered a long time (though of course the ages of the kids might help decide that). Also, she's only trying to get custody of one child - why not both?
These are, as I see them, rather large hurdles to overcome. I do not see a Judge ruling against Mom in this instance, and that's why mediation - where grandmother has the chance to persuade Mom outside of the court-room - may actually be of benefit to grandmother.
See where I'm coming from?
An alternative would be to stop the grandmother's suit and focus on your son's custody case.
One other thing:
Dad needs to be filing contempt EVERY SINGLE TIME Mom disobeys the court order.

