If she is getting around okay maybe move her to a separate layer flock if you don't want to eat her, to keep you from accidentally saving eggs from her to set. Its a shame she has the nicest topline in the bunch.. that's the way it goes. It's probably best not to breed her though. Marans are a heavier breed and they need good strong legs at their base to support their weight.
Definitely let the man show you his methods so you can learn. My neighbor helps me butcher my birds because she is better at it than I am. You can also check out the broomstick method of dispatching chickens, I think there might be a video on youtube. Basically you just hold the chicken upside down by the legs and rest its breast on the ground. Put a broomstick down over the bird's neck and step on the broomstick with your foot. Lift the chicken up by its legs and pull. It's fast and I imagine fairly painless as it breaks the bird's neck very quickly. You can then hang the bird after it has finished flopping, cut its head off and bleed it out. Wow this sounds very graphic when I put it in writing, but you get used to it. I don't like to kill the chickens either, so my husband does it. I only kill chickens in emergencies when my husband isn't around and one is suffering. My husband is a bit more old school in his techniques. The old wringing of the neck method still works for him just fine.