Listen man, I sympathise, I really do. But the dude your 12 year old nephew loves is not only a serial abuser of women, he was proud of it and he boasted about it. He didn't try to change, he didn't try to repent, he didn't try to make amends. He beat, abused, strangled and hurt women who trusted him. And, again: He bragged about it.
This could be a very good chance to explain your nephew that not everything is easily fixable by saying "I'm sorry". That there are things that aren't fixable and that you would have to show year long and incredible effort to even come close for people to forgive you. And even then you have no right to expect them to. And, most importantly in this case: That this man showed absolutely zero willingness to do so. That he can still enjoy the music and appreciate the music for how it helped them - but also be aware of the fact that the man behind the music himself is the worst kind of bully there is.