Your friend cannot force this person out without a Court Order. If she leaves of her own volition, fair enough. If not, he will need a Court Order. In the same claim he needs to claim for the unpaid rent and mesne profits.
The tenancy agreement should include provision for inspection. If she will not allow it, you need to include this in the proceedings. If there is such a term, you need to make it clear to her (in writing, by signed for delivery) that an inspection is necessary. If she fails to comply there is, however, little you can do (subject to the terms of the TA).
I suggest your friend finds an interim solution until he can get rid of his tenant.