unfortunately i have no idea why she decided to change her mind, but i have heard stories like this before. you are going to have to separate them because the stress could cause more problems for the boy than a wounded tail.
if the wounds are pretty bad then you might have to set the boy up on paper towels until they heal just so your substrate does not irritate them or cause them to become infected.
the only thing that i can think of is to keep them apart and maybe try again in the spring, monitored of course. how big is their cage? perhaps they need some more space?
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vonnie
***There is no pleasure in having nothing to do; the fun is in having lots to do and not doing it. Mary Wilson Little ***