Is fostering a child allowed in an assured shorthold tenancy?

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We have a landlord who is complaining that her tenant is in breach of her assured shorthold tenancy agreement and must vacate because she has a foster child and is therefore running a business from home! and apparently making the landlord's buildings insurance null and void! The tenant does work full time elsewhere, the foster child is not her main income and she was referenced on the basis of the main job - the landlord also met her and her foster son prior to the tenancy commencement but is now asking for our agency fee to be refunded! Any thoughts from anyone?