Tenant's pregnant girlfriend claims she is joint tenant

Possession - Section 8 and Section 21

I have been contacted by the girlfriend of a tenant of a shared house. Although she is not the tenant, she has been co - habiting with the tenant, sharing his room at the property. She has become pregnant and I can only assume she wants to be given a council house and been told she will be given one if she has a baby and is homeless as she has requested that I serve a section 21 on (them - she claims she has been paying the standing order for the rent so this means she is a joint tenant even though she has no contract). I served a section 21 on the tenant (her boyfriend) on the day he signed the AST tenancy agreement and this expires at the end of this month. Although the tenancy agreement says the tenant is not allowed to have guests stay at the property without written permission, I did not take action when she moved in with him. Does this entitle her to claim that she is a joint tenant? If so, do I need to serve another Section 21 on both of them at the end of the month and wait 2 months before I evict them both, or can I use the original section 21?