I think it is highly likely that when she visits her father, with her two little girls, that she may be sleeping with him, so that he doesn't abuse them.
It would be unlikely that she would confide that much information to you, because you would feel even more obligated to call the authorities.
That the abuse for her started when she was a little girl, and she has her own little girls now, puts those two little girls in a very high risk situation.
It is highly unlikely that the mother is the only victim over the years.
I think you have an obligation to report this man. It is of course about your friend, but this man has to be stopped before he abuses these children.