To separate or not to separate, that is the question.

[Weehopper]

We always single mated our hens, but would leave hen and cock together until the hen started setting. Then , usually, the cock was removed. The hen was left to set in the pen, then after chicks hatched and grew for a few weeks, she was turned out with the rest of the flock. She will protect the babies, unless she is a particularly shy hen.

 

[donrae]

I would second the poster who suggested simply getting fertile hatching eggs and placing them under the hen. Bringing in a rooster to an established group of hens who have never had a rooster can cause a lot of issues in the flock dynamics and throw laying and broodiness into a tailspin. Plus, you have the risk of disease, etc.

I'd simply put the word out on whatever local classified ads you use that you're looking for fertile hatching eggs of good layer breeds. If you're not set on a specific breed it should be easy enough to find some local.

Or, let her brood and get day old sexed pullets to place under her. Hatching your own is going to yield 50% cockerels, do you have a plan for those birds?