With still reasonable assumptions you could justify just about any numbers you want for this exercise. I assume you don't want to discourage your friend, so you may want to be a little more aggressive on the withdrawal rate, and maybe more optimistic on the investment returns as well. There will be plenty of time to change the plans over the next 20 years if things don't turn out that well, and she'll be better off if she's been putting together $200 a month starting now, than if you tell her she needs $600, and she decides to forget the whole thing.

If she puts away $200 a month and beats inflation by 5%, then she'll have $123k in today's dollars in 24 years. At an 80% success rate over 20 years you can take out about 1/15th a year. This gives her $8.2k + 10.6k SS, and she has 18.8, which is pretty close to her income now after putting $200 a month in savings.

Frankly, I wouldn't give this plan much of a shot of working out exactly as presented, but the reverse mortgage could help things out in the future, and if she re-evaluates in 5 or 10 years and needs more, then it will be easier to ramp up savings from $200 to $400, than from $0 to $600.