Something I noticed which might give you some trouble (separate to the CT issue) is the outstanding rent. She signed a tenancy agreement saying she'll be in the property from x date but told her house mates she wouldn't pay until she moved in, 2 weeks later. Whilst that's all well and good between mates, in a situation like this where he's now kicking up a fuss, she hasn't got much in the way of a leg to stand on on that one, as the tenancy agreement will say she should be in the property at x date.
Again, I had this issue arise with my former housemates. One housemates family lived in London and he moved out in July. He tried to not pay rent saying he didn't live there any more, until we pointed out the contract was until August, so he was paying if he liked it or not.