I think we need to clear up the term “house in multiple occupation”. If the property was a HMO, then the liability for the Council Tax would fall upon the owner (Council Tax (Liability for Owners) Regulations 1992) and your client would have to appeal against the decision of the LA to hold her liable.
http://www.legislation.hmso.gov.uk/si/si1992/Uksi_19920551_en_1.htm
However, if she is was a joint tenant with the other three residents then they would each have what is termed “joint and several liability” which in effect means that as they all had the same interest in the property, they can all individually be held liable for the Council Tax due on the property.
