As you have seen intuitively, this indeed is the case. The grandmother is correct with her intuitive sensing that the granddaughter is not only at risk but has been abused and this is the reason for the change in behavior for the worse. Also, as you saw intuitively, it is the biological father who is the culprit here and not the step-father.
So whether or not to share this information is for you to decide. There can be many complications ensuing from the divulging of such details. This is a particularly unfortunate situation because there are two men in her life, so to speak, and so confirming there is abuse going on, points two fingers and is unfairly prejudicial against the innocent party. On the other hand, there are ethical and karmic issues as well in having knowledge of harm being done, but taking no action when action is possible.
In this case, you personally are karmically involved because you are the only one who can obtain outside confirmation. It is not legally enforceable, but it can be a tool to cause action to take place on the part of the grandmother. Whereas otherwise, without your corroboration, she might shrink from her duty as a guardian and watchdog looking out for her granddaughter’s best interests and remain silent and then the abuse can grow, and grow, and grow. So one way to do this would be to approach the grandmother in stages and break the news gently and go forward bit by bit with information, having introduced a discussion of options and cautions about the consequences for her as well in making a dark accusation of this sort. So that said, we will leave this in your hands, as this is your project you have launched, to sort through the options available to you and make your decisions in how you wish to proceed.