I think @lemonpie35 is, in one sense, another one of her father’s victims. You are supposed to be able to rely on your parents, to trust them 100%, and now she is mourning the father she thought she had, the trust she thought she had in him, which he has so wickedly betrayed.
When he chose to watch videos/view images of children being abused, he did so with no thought at all for the effect that it would have on his family, if his crime was discovered. He has shown himself to be selfish and to have no regard of the feelings of others around him - even his own children.
She is in the horrible situation of having two fathers - the loving, trustworthy, decent one she thought she had, and one who is a paedophile - and it must be horrendously difficult to deal with.
Plus, half of her, genetically and in terms of upbringing, comes from him, so maybe rejecting him feel like rejecting part of herself. It could also be that she needs to minimise what he did, because the alternative - that she is half him and he is what he is - is too awful to contemplate. I can understand that.
BUT her dh is 100% right to refuse him contact with his grandchildren. A man who has done that is not safe to be around children.