I think you have to take her financial state into account.
She is doing what she can afford and by fixing her females at least they aren't producing any more kitten litters. I understand the males can/will still roam and breed other females, but she needs to take care of what she can take care of, first.
Can you offer to pay for fixing the males for her, as an incentive?
Here is another angle for argument:
The more cats that arrive UNspayed/neutered and she feeds, the more kittens will be born and brought to the food. More and more kittens from the litters will survive and eat the food she provides until eventually there are more cats than food, then the numbers will begin to decline as some do not get proper nutrition and their health will suffer.
Yes, it is nice to just feed them and feel good about it, but you also have to be aware of what your limits are, because by placing food you are changing the dynamics of their survival.
Good luck, I know how frustrating it can be.
When I came home from the hospital I developed an infection and called a neighbor who is a nurse at our local Doctor's clinic. I was very thankful that she came, but we started talking about cats because she saw mine and she made a comment about how she feels all female cats need to have a litter of kittens before they were spayed.
I decided to not say anything. First, I just didn't have any energy. Second, I did not want to alienate her when she was so kind to drop everything at home and come over to me when I called. Finally, I don't think I could have got through to her about the importance of spay/neuter in the short amount of time I had and with my panicked mode of thought at the moment because of worry over my condition.
I may try again later, when I see her at the clinic for my checkup. I'll be prepared and show her how 1 cat, having 4 kittens, 2M/2F, and all the females having litters with 2 females each, how those numbers will quickly add up to homeless cats.
Black and White Mathematics. Maybe that will get through?