I'm not sure what the Catholic view is, but it's too late to pray for her once she passes on. That die has been cast.
You’ll notice I agreed with you that “the die has been cast” and the departed person is already, and irrevocably, saved or lost.
However it is not too late to pray, for two reasons: 1) we never know whether a person has gone to hell, but even if they have, the prayer won’t benefit them but it will benefit us. 2)if they are undergoing a period of purification (purgatory)-— which, I think, most people do before they enter heaven, since “nobody’s perfect” -— our prayers can console them and hasten their entry into heaven.
The Apostle Paul says (1 Timothy 2:1) “I exhort that supplications, prayers, intercessions, and giving of thanks, be made for all men.”
The Jews prayed and made offerings in the Temple for the dead, because of this understanding that life is not ended, only changed. Most Christians understand “all men” to include the living and the dead, since Jesus also teaches us that all are alive in the eyes of God.