Hi Mike:
Short answer: don't do it.
Longer answer: there are too many potential issues with this, that can cause confusion, anxiety and/or stress in an elderly person. The WiFi access point may be clogged with other people's traffic, resulting in voice drop-outs, distortion, or hang-ups. Google Voice cannot call 911, and you'd have to add that, and periodically test it. Google Voice's features, such as voicemail, are potentially too complicated for her.
A better solution is to simply port her land line to a prepaid cellular phone service with however many minutes she will need per month, and leave the phone plugged into a charger near her bed. If she has any physical impairments, such as poor vision or hearing, consider getting her a "senior-friendly" cell phone handset, such as the one offered by Consumer Cellular, or the "Snapfon".