Hi Kevin,
It would be so sweet of you to offer to add onto the coop and/or expand the chicken yard. I imagine the woman would really appreciate it.
How did the chickens look? Were they healthy? If they looked picked on or unhealthy in any way, you could use that as the reason you want to help her increase the space. If not, I suspect just telling her you'll be happy to help will be enough. It sounds like she's aware it's not enough room since you mentioned it and she said she hasn't had time to tackle the issue.
You are a very kind person. She's lucky to have you as a neighbor. Let us know how it ends up.
Take care,
Penny