Your friend having a British passport doesn't automatically mean that she's a British citizen. Other categories of people (such as British overseas territories citizens, British overseas citizens and British subjects) can qualify for British passports but aren't 'full' British citizens.
So your friend should check whether she qualifies as a British citizen here:
https://www.gov.uk/check-british-citizenship/born-in-the-uk-or-a-british-colony-before-1-january-1983
(The rules vary and depend on her date of birth).
If your friend is
definitely a British citizen, then she automatically has a right of abode in the UK. [Section 2(1)(a), Immigration Act 1971 applies].