Well, on the one hand, Joseph (who didn‘t have his mother‘s respect for the clergy) seems to have put the fear of God in that priest, and MT never said she didn‘t believe it, but...

...on the other hand, she did say that it‘s a comfort to her FS went to mass that day (it being Sunday) and took communion. Which could be read as meaning just that, or could mean she‘s somewhat suspicious of the „there were still life signs“ story but tells herself that there wasn‘t much sinning happening between morning, when FS had communion, and evening, when he died, so he still was in a good spiritual state.