They say 4 sq ft per bird inside the coop, so 1200 sq ft.
Ah, just saw you live in FL. I think that 4 sq ft inside is for locations where birds may be locked into their coop/house for periods of time, such as bad winters, etc. And that may be the case where you are if you get the hurricanes coming through or bad storms regularly. If not, I think you could go down to maybe 3 sq ft per bird, as long as they had plenty of outside space with a large shaded area that was sheltered for wind. I've heard 10 sq ft per bird is the minimum you should have in the run area, so you would need 3000 sq ft for the run area.
With this many birds you'd have to think through many issues such as provision of feed and water, unless you want to make it your day's work just feeding and watering them. Also have adequate storage for feed and other necessities. And predator proofing an enormous run. Wow, my head hurts thinking about it. And here I was, thinking 10 was a lot.
I would start smaller and work up. That's a whole lot of birds and will need a lot of feed and room. You need to make sure you are zoned for that size of operation. Is she wanting to sell eggs? Meat?
sharon