Dearesr gismo,
You and your family have my deepest thought & prayers.
May I ask how old she is? Does she practice a particular faith?

I understand you not knowing what to say, it is something that is not taught/
talked about in the open. So how would you know. The best thing you can do
is listen, let her talk, ask her how she feels and let her express what she wants
or needs during this time. If she is'nt one that talks thats okay too. You could
take this time to tell her how you love her and tell her she is not alone. To keep
her free from physical pain. Honnor her by telling how much joy she has spread upon the earth. You have a husband because she was a mother, if you have children, she was a grandmother. These are things to celebrate.
If there was/is any discord this would be the time to forgive any harshness.
Some people die suddenly or alone and don't get this time.
We are all humans born of light/energy and in the end this is what we return to.
Gismo, all you can do is hold her hand, caress her head, keep her calm and free of pain and tell her there is nothing to fear, she is not alone.
If she does have a particular faith encourage it, whatever brings her peace.
With all due respect I hope I did'nt say anything to offend you.
Prayers and Hugs,
Happiedasiy