Since the two of you know each other quite well and this is a private conversation, I see no need to stick with "traditional" lines of condolences. I speak from personal experiences in which friends supported me through difficult situations in untraditional methods, since that was what I needed at the time. As they teach in client-centred therapy, the important thing is to support the person in need of help. In this case, you illustrate a person who has had a very difficult relationship and who therefore feels liberated by the person's death. I felt the same when my mother died, since she engaged in lifelong emotional abuse. Friends allowed me to say that the best thing she did for me was die.
Yet it also felt like the bottom dropped out; a restriction and oppression that had hemmed me in all my life were suddenly absent. One reels from sudden release of that, and feels guilty to express relief and liberation. Thus, the person who "celebrated the death" of this relative will most likely need reassurance that their response is okay. For all these reasons, I suggest that the important thing is that your friend feels you understand and that you don't judge.
I forget the exact words my friends used for me, but the following examples are the spirit in which I have been supported. To sign off the conversation, you could say something along the lines of:
- One doesn't normally say this to a person who is going to a funeral but in your case, this will put closure to a sad chapter and mark the beginning of the new.
- I am happy that life will be better for you.
- I look forward to hearing what life is like in six months when you get used to this new situation. I bet it will feel so liberating in ways you can't yet imagine.