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It is possible that your granddaughter is displacing her frustration onto both parents by a way of acting out disobedient/rude.
Her mom have got to address this right away, so your granddaughter does not make a pattern of it and get away with it.
If there is things she can consequent her with when the behavior is inappropriate, then that may help shape some more positive behaviors. Of course, all disciplining/consequenting ought to take place on a consistent basis.
It may be a good idea for her grandfather to point out the positive her mom does for her. He may want to omit what the father does not do (kids are very observant and she may have figured this out) That way he won't also say negative things about one of her parents.
Assigning her some age appropriate responsibilities would be good whether it is chores, doing her own laundry etc.