Hey, give her fat, not fiber. Fiber is not needed at all. If you have some coconut oil, give her some of that in some soft eggs or something. Lard is also fine. Fat will solve the issue IF she's actually clogged up and it can't hurt this time of year anyway. Solves it for humans as well. I'm not sure it's what you think it is. I've never thought any of my birds were constipated.
You may want to read this, not to worry you. I see urate poop in old birds a lot, though.
Watery Urates Poop
If your chicken produces droppings that are very white and runny regularly (the urates is runny), this should be monitored closely as it can be an early indication of a kidney problem- more so common in older birds.
If you happen to come across an abnormal chicken poop, it is important to note whether it is an isolated incident, or one that is recurring. You should also check for other symptoms in your chickens that may signify illness or disease, such as: weight loss, loss of appetite, lethargy, sullen appearance or a drop in egg production. If these are established, appropriate further treatment should be undertaken.