Our first step would be to contact the Tenant, and reiterate the conditions of the Tenancy Agreement, advising that the type of behaviour reported is unacceptable. The Tenant is responsible for the conduct of others living in or visiting the property.
(Should I talk about Section 12 of the ASB Act 2003???)
Evidence of any anti-social behaviour would be recorded, with dates, times and places of the incident(s). If all attempts to resolve the matter amicably fail, then it may also be helpful to report the problem to the Local Authority or consider ‘mediation’.