I do a lot of natural light photography, or ambient light photography. No flash, no reflectors, no scrims, just the body and a lens. The first reason is because I am basically lazy. The second reason is that it gives me away if the flash is going off. The third reason is that I am frequently too far away from my subject for the flash to reach with enough power to be useful. Then you get into issues of weight, size, convenience, speed. I usually have at least one speedlite in my bag and I will get it out and use it if I think the benefit is there.
She owns two 600 EX RT’s and a 580 EX II. At a store that’s local to her, a single 600 EX RT is USD549.00 and they have stock. So, she owns about $1500 worth of speedlites. I’m guessing she has an idea what to do with them. All together I have 7 speedlites of various vintages, 4 studio strobes and several continuous lamps. I still shoot easily ten ambient shots for every shot that uses “lighting”. But, I shoot everything. If I just did portraits I would probably use lights a lot more.
She has nice photos. The catch lights I see in her outdoor shots look like sky, not strobes. Bare flash gives small bright spots. Softboxes and umbrellas give shapes that match the modifier used. If the catch light is lighting the whole top half of the eye, it is probably sky. If your subject stands in the right place the sky is reflective enough to give catch lights when the sun is behind them.
I have heard the line “When I hear ‘I only shoot with natural light’, what I think is ‘They don’t know how to use a flash'” and it makes a degree of sense. But perhaps the thing is that they can get the result they are looking for without a flash. I see her skys are usually blown out. If she wanted a deep blue sky, flash would be a way to achieve that. So, in some cases, would be a circular polarizing filter. Flash will work if you have a powerful one and your subject is 50 feet away. A CP filter will still work if you are using a 400 mm lens and your subject is 500 feet away, flash might too but you will need a lot of it if you want to do anything more than slight fill.