I know this doesn’t meet the parameters of FOSS, but in this instance I’d discard privacy concerns and just get her an Echo or Google Home. They’re cheap, and bring able to call out a help request is better than a button she can forget to carry or wear. Either is capable of hearing a command in a 2br apartment, and have SOS ability.
I have a failing MIL in a similar situation. She’s living alone, isolated, not near any family, and won’t move. I gave her a Google Home I had in a box from a few years ago, and it is a godsend. She talks to it, it talks back, and she can make it do things she’s not capable anymore of doing with technology. But most importantly, she can call any of us by telling the Home to do so.
I wouldn’t risk a cobbled-together solution, personally, and right now the OSS technology stack for voice recognition is (a) not inexpensive, and (b) futzy. I have no doubt it’ll get there; there are already some promising devices, but that integration with calling and SOS is the key.