Is she still conscious ? Many times by this stage they are unconscious mostly often due to pain killers or other drugs. I only ask because her openness to the final sacrament depends on her making a decision.
Its never wrong to consult a priest though especially about death.
Beyond that there is simply prayer. If she is a devout protestant then I would not pray the Hail Mary or other clearly catholic prayers in her presence. Even though they appear unconscious hearing is often still very active. Instead pray prayers that will both comfort her as well as protect her.
Psalm 23 and many others
Protestant hymns are often prayers and beautiful. Softly singing amazing grace, in the sweet bye and bye, shall we gather at the river, nearer my god to thee, on and on and on. The songs are less objectionable to the dying and yet are also prayers.
Its hard, no matter what you do you will question if you did enough or the right thing.
I still pray the Catholic prayers regardless of the faith of the person but I pray them quietly next to them. But I try to be sensitive to their wishes.