Tenant moving in a partner

Tenancy Types and Management

My current tenant is a lady in her 50s, she's nice, professional and reliable. She has asked me if she can move her boyfriend in (who currently lives about 300 miles away). In principal, I have no problem with this but my initial intention was to include him in the AST and amend it accordingly. However, she says she wants to 'retain her rights' as the tenant, should their relationship not work out. This seems acceptable to me as I feel it may complicate matters if I put him down as a joint tenant. Is this the right action and does he have any 'rights' as a tenant etc, should their relationship not work out?