@MajorMajormajormajor
You can also try #ChildMonitor

Child Monitor allows two Android devices to act as a baby monitor. The first device, left in the room with the baby, will advertise itself on the network and stream audio to a connected client. The second device, with the parent, will connect to the monitoring device and receive an audio stream.

GitHub - enguerrand/child-monitor: Android-based Child Monitor

Does something like this look like it might work? It requires having an extra phone?

Unless you're setting up your own stuff like Frigate already suggested here, I would suggest one without Internet connection but wireless video monitor. I used a eufy E110. Signal worked all over the house and picture is clear. I didn't think any of the COTS solutions with an internet connection and app could be trusted TBH.

For inspiration I'll describe our setup:

We have a TP link TAPO camera, it sits on a VLAN without access to the internet. We can access and watch it through home assistant. There's a custom integration that allows for some control of the camera including motion control, motion and noise detection, though the basic integration should be fine by now for just the video and audio feed.

Since we have some notification set up when the motion or noise is triggered, we don't need to have the video feed open all the time (it even pops up on our Kodi TV). Though, we do use an old spare android phone as a dedicated baby monitor, but we're not tied to using just that one.

This is stationary for our home though and the initial setup of the camera does require internet connection, but after that... It all lives locally.

We also do have a cheap TrueLife video baby monitor, closed loop, with a dedicated receiver unit, but that is more of a travel solution. No fancy function, just video and audio feed from the camera to the parent unit. The range of this thing is rubbish though...
And when we don't need video feed, we use regular Motorola PMR walkie-talkies, one of them (on the child side) in VOX mode. Any baby monitor can only dream of its range and battery life...

So, that's our setup in a nutshell. If you'd like to know more, feel free to ask 😉

If you're more technical or willing to learn I would recommend Frigate https://frigate.video/

I think video is very unnecessary, audio is the most important and there are great ones which don't use the internet or even WiFi but a different radio signal like https://a.co/d/3YAxSzw

If she really can't do it without video then I'd buy a ubiquity one because they work without internet connection if you set them up like that. Then only you can access it. But they are quite expensive especially the ones with 2 way audio.