She's allowed to return to the
UK until the expiry date or her current visa. But in fact, if someone returns within a few months of their visa expiring, immigration officers at the border may ask them more questions than usual, especially what are the person's intentions after their visa expires.
That being said, it's usually not recommended to travel abroad with less than 6 months remaining on someone's
passport. Depending on the purpose and length of stay, some countries requires 6 months, other 3 and some other even less. If it's your girlfriend case, she should check with the Spanish authorities, but if it's only her visa that expires within 6 months, she should be fine.
VisaWise | Immigration ConsultancyOISC Registered Immigration Advisers
London, United Kingdom
info@visa-wise.co.ukhttp://www.visa-wise.co.uk