There's a few different things going on here:
First of all, grandparents have no legal right to "take over" their son's parenting time. Grandparents can try to petition for custody or visitation rights separately, but their burden is a very high one. They would have to sue both of the biological parents, and prove that both parents presented a serious risk of harm to the child (if seeking custody) or that they failed to act in the child's best interests by forbidding visitation (if seeking parenting time). The law normally presumes that parents do act in their children's best interests; the grandparents would have to rebut this by clear and convincing evidence - which is a pretty high evidentiary standard.
As far as drug testing: you can't "make" the guy take any drug test, yourself. But you can ask the Court to order a test - or, more likely, have the Court order a custody evaluation, and ask that the evaluation include a drug and alcohol screen. The evaluator will find a doctor who will provide what testing they think appropriate, based on the evidence they find.
Your girlfriend should be consulting in private with an attorney if she's going through a case like this. This board really can't be an adequate substitute for that.
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