Whatever new phone you get will most likely have a non replaceable battery. Make sure to keep it charged. Running a Li-Ion battery into the ground is what kills them quicker than anything else.
I cannot confirm or deny... But new phone batteries are amazing. You can go a few days on a new one if you're not browsing a lot, playing games, or on GPS/maps a lot. I'd imagine that she's not a "power phone user" - so can probably get by with only charging like an hour or so every day. Or even every other day for a few hours. (This is my experience with my fairly new Pixel 3a - and I use it a fair amount)