Several points, Mo, but I'm far from being an expert so this is just common-sense suggestion.

She could start by finding her comfort range. Get a cheap keyboard and see which white notes you can sing. Make a note of your lowest comfortable note and the highest. That is your range. If that comes to less than an octave (7 notes) you're in trouble and need a doctor's advice.

Regarding singing along in church. If the notes are too high several options.
Drop an octave. Practice dropping an octave mid tune. That way if you can't sing the high notes you can drop into a more comfortable octave.
Try starting the hymn in a lower octave if you know it's going to be too high for you.
Lots of practice at home till you are happy with a particular tune then play that tune back and try some simple harmonies lower than the note. Trial and error is fine. It's all good practice.

Regarding changing octave mid tune, for quite a while now there have been some female folksingers who do this all the time even when singing solo. When I first heard it I was a bit taken aback, but now it sounds quite natural to me.