I don't know if I'd be a good gauge of what other people would want, because I'm pro choice for a lot of people. But I'd want to know why they are taking their own life, that it's not impulsive, that they've made peace with and are absolutely sure of their decision, and that this has been building for a very long time, because the thought that they took their own life over something that could have been resolved would really tear away at me, not as much as losing them but it would linger for the rest of my life along with the loss. I'd want to know if there's something I could have done, if they were angry at me.
I think if they've been a source of love and support in your life definitely talk about that and emphasize they couldn't have fixed this because it's something completely outside of them that lead there. What riamu mentions, recounting good memories with them, is also a great idea.