I've had buttons for a couple of years now and I've had a few instances of females acting aggressive towards males in that time.
First was with my first group of buttons - 3 females and a male which were put in my budgie aviary. They hadn't lived together previously and some of the females were chasing the male a bit for the first week or so. Then they settled down and lived together peacefully for 1½ years, hatching and raising chicks together and so on.
Then I moved to the other end of the country leaving my parents to care for the birds, and apparently they over feed the budgies, meaning shitloads of seed end up on the aviary floor. The buttons prefer eating seed over their more healthy diet of chick starter, and so, they do. But this means they do not get the protein (or vitamins) they need and so they started feather picking.
One female still has her feathers, the roo and two other females have partly bare backs. I'm not certain it's only the one female picking, but I haven't seen others do it. I've tried separating her (putting her in a different cage within the aviary) when I was visiting my parents and this has resulted in some regrowth of feathers in at least one of the other hens, but the captive one has managed to escape from her cage twice now so with me only visiting my parents every 1-2 months it's not really efficient.
They should probably all be housed separately until they regrow their feathers, but my mother won't have animals inside and it's winter now so I dare not prevent them from sharing body heat with their bare backs outside in the winter.. It's been going on for half a year now, so even if I do separate them and let them regrow their feathers I think they are likely to just start over again when reintroduced. And my parents are still over feeding the budgies, despite me telling them twice that this is likely the cause of the problem.
A different instance was among chicks in this aviary. I had a large group of 15 chicks at one time and I moved something outside the aviary, causing a panic. The chicks were 5-6 weeks old and one male hit his head on something, causing him to bleed. Within seconds his siblings started pecking at it. They didn't look aggressive or anything - they just couldn't leave the big red drop alone. So I placed him in a separate cage in the aviary for a week or so and there were no issues when I reintroduced him. But they probably could have killed him if I'd left him there.
Last instance was second generation out of the aviary - I have a pair where the female was a chick from the aviary and they had chicks in a cage in my living room. At about 6 weeks one of the female chicks turned on her mother. I separated her with one of her brothers thinking as it was a female she was aggressive towards she was least likely to attack the brother. She did attack him though, but not as violently as she had with the mother and she calmed down after a couple of days.
So, based on this and what I've read elsewhere, I have 2 ideas as to what caused the aggression. Either, as 71st suggested - the male got hurt and the female started pecking and didn't stop. Or, you are feeding them something unsuitable causing the pecking. Of cause the female could also just have gone territorial for no apparent reason like my female chick towards her mother, but it's easier to fix the other problems, so let's exclude those first.
What do you feed them? Everything, from the nature of the 'greens' you mention to the smallest treat, to what's always available to them, to the amount of each treat and to the amount of protein in their regular feed.
How big is their cage? Height, width and length and material of the top? Anything he could have been hurt hitting? Any dry blood anywhere in the cage?
Anyway, you are doing well by keeping them separated, but it would be best to keep them where they can see each other. And as the female was the aggressive one, I might actually remove her from the usual cage and let the male have it - that makes it less likely that the female is aggressive when they are reintroduced.
If possible, the best solution is probably to place a mesh separation in their usual cage to keep the female away from him while still having them as close as possible. Otherwise, buy or make a wire cage to keep the female in while he heals and place it right next to the usual cage.
