The tenant has a right to quiet enjoyment and has the right to have at least 24 hours notice of any appointment. The tenant also has the right to refuse access to the property and an agent/landlord only has the permission to enter the property without the tenants consent in case of an emergency.
Having said that the tenant is also bound by the terms of the agreement to co-operate with viewings for either sale or re-rent of the property.
Is the property managed by the agent or has she been dealing directly with the landlord throughout her tenancy? If the property is managed, I would find out if the agent is accredited with ARLA or NALS, if they are I would write a letter to the Director of the company and send it recorded delivery stating that she will only allow access to the property with 24 hours notice or she will raise an offical complaint through the scheme that they are accredited with.
If she has been dealing with the landlord I would speak to him and tell him that you are not trying to be awkward but you have rights and are not happy for his appointed agent to waltz in willy-nilly.
If she is concerned about her belongings she can change the locks but she must provide the agent/landlord a key at their request (It might make them think twice.
Hope this helps
Good luck
Mrs Keyser