This has happened to us too. Main problem when she's sitting is that all her chums come to sit in with her and lay their eggs in her nest. This means unless you can recognise the fertile eggs from everyone else's, and remove the aliens, you have so many eggs in there, which will either not hatch with the fertile batch, or the good ones will be shoved out, she won't be able to cover them all, and the whole lot can fail. Even if some chicks hatch, the other eggs would be rotten by then, likely to break, and you have a nest full of stinking rotten egg

So far we have had to leave these hens in the main house with the others and remove the alien eggs (wearing thick gloves against pecky hen). As soon as the chicks have hatched, but before they start to move around, we move hen plus brood into a small broody ark, where they stay for a while. Then we put them into a larger chick ark, and finally let them run free - if it's winter or we're worried about airborne predators then they go into the polytunnel. We've occasionally ended up with 2 mums of one brood, and initially they share nicely, but one will eventually take over from the other.
So if you can get her to move with her clutch to a broody coop then do - saves a lot of hassle, but for us it's not always worked - you find the hen brooding nothing in a corner and freezing eggs left on their own in the nest.