How to comfort the families of dying patients in English?

In fact, the best thing is to be with him or her.

Say it if you want.

It's gonna be okay. His/her God and guardian angel will help her through this difficulty. God knows how much you love him/her, and God won't take him/her away from you.

You need to be happier. He/she needs you to bring some happiness to his/her life.