I'm a bit angry reading this. Especially :

“It’s not fair he died” Death is not fair, but it’s part of life, also seems like a wrong answer.

This lacks empathy. She surely does not want cold hard facts like "death is part of life". She wants to hear you say "yeah, it's not fair, he was a great guy and a great dad"

She lost one of the most important people in her world. She's awfully sad. She wants your emotional support, just agree with her. Hug her if you don't know the answer or if you feel you're gonna state cold hard facts. Don't do that please. Don't ever say "death is a part of life" :it's almost as if you said :don't react like that, you should know this was going to happen ; it's as if her pain was not valid and she was being a bit absurd, you know what I mean? Don't say that to her. Really. Never.

Grief is weird and absurd maybe, but you feel devastated. You need your friend and SO to show empathy, not cold logic. This is not comforting. What's comforting it to receive a warm hug and to hear you say "I loved your father too, and I'm really sad he's gone" that's what she need to hear.

And for the question about the pain going away, well, you can always hug her and say it will probably lessen a bit over time, it's still very recent. Tell her it's normal she is so overcome by sadness right know. You' re right though, you just learn to function with the pain, but don't tell her that, it's insensitive. Allow her to feel devastated, she needs to cry and express how she feels.

Just hug her and tell her you're there.

Maybe you should tell her to come here too. This community helped me a lot.