The egg laid in the others clutch will not hatch at the same time as the main lot of eggs, so why not swap that for the marble egg so that hen has at least something to hatch. Alternatively give her a clutch of fertile eggs bought in. I thought marble eggs were just to keep a hen broody while you send off for fertile ones. If you take away the marble egg she will continue sitting on an empty nest, for a while anyway. Just make sure she has plenty to eat so she doesn't lose condition and can then start laying again when she goes off being broody.