One other option I forgot, rather than a generator and all that other stuff, have your sister have an electrician add a 30amp RV outlet and buy a 30 amp 50' RV extension cord. Bet it will be cheaper than generator + the other stuff.
Even at list prices, the generator, watchdog, 50 foot cord and 15A to 30A dogbone is less than $1200. and it is portable.
Not to mention the generator is HOPEFULLY an optional purchase, as once the proper cords and power monitor are in place the problem will be solved and/or diagnosed.

Or consider what $$ would be needed to fix the onboard instead of ~$875 on a new generator.
So really the out of pocket is around $300 for the monitor($100), extension cord (50 foot is ~$90, 30 foot ~$70) and dogbone adapter ($15).
I would spend ~$300 in 6/3 wire to put an RV outlet 55 feet away from my panel downstairs, and thats with me doing the labor. Plus supplies (breaker, pole, outlet, cover box) Add a 'Qualified Licensed Electrician', and I bet you could see $1200 easy.