My mother would watch the tv at a far higher volume than needed. I was convinced she could not process what the people were saying, and kept raising the volume in an attempt to remedy this.
Unfortunately I think the answer to your question is she requires night time supervision, which is not what you want to hear.
One idea would be to put the plug of the tv into a timer, so the tv was not capable of working between 10.30 and 7am. A box would need to go over the power point to prevent tampering.
It’s not a good idea, because she would not read the large note you leave explaining this. She will also most likely phone you to report the tv not working.
You could perhaps obtain an old portable tv with poor volume level ( or take the case off and muffle the speaker with foam). If that was elsewhere in the room and worked all night would she use that instead??
I am afraid that is the only idea I have at present! I hope others have more ideas to help you with this one!