non payment of rent by one tenant in shared flat

Rent Arrears

My daughter lived with two other students in a student flat - July 2008-June 2009. The landlord has just returned the holding deposit this week after repeated requests. He maintains that one of the tenants owes a substantial amount of rent. (I have been told it is 6 months)
The tenancy agreement is worded 'jointly and severally liable'. Each tenant has a signed guarantor - parent in each case.
Today my daughter has received a letter from the landlord stating that despite his repeated attempts through letters and phone calls to the tenant who owes the rent, he has had no response and therefore will issue a County Court Summons in 7 days to each of the three tenants.
My first question is - what can my daughter do about this,
and secondly - why doesnt the landlord issue the County Court Summons against the guarantor of the tenant owing the money. Surely as guarantor he has signed to say he will be liable for anything his duaghter defaults on. Is the landlord taking the correct course of action? Thanking you.