I think this is an issue that needs to be dealt with - and urgently - by the child's father. As her parent he has a direct responsibility for his daughter's welfare.
If you have not already done so tell him what your DGD said to you. He should chat casually to his daughter to see if he can find out what she means when she says that her mother's partner has 'forced' her to eat.
It could be little or nothing, perhaps being told she cannot have something nice like ice cream until she has eaten most of her main course, but, more dangerously, it could be the start of a pattern of developing child abuse
Tread carefully with your DGC but otherwise this could be a warning sign of potential child abuse and should be acted upon promptly.
He has done it! The toolmakers son has resigned!
