Protocol for writing a recommendation letter for someone you only know on a personal basis
Graduate admissions panels are not (generally) looking for personal references. They are looking for academic references.

Tell your friend to find another letter writer; nothing you say about her wonderful personality will help her get admitted. At best, it would be a wasted opportunity for her to submit an actual academic reference; more likely, it would actively work against her (looks like she's clueless, or such a bad student that she can't get anyone to write an academic letter for her).

Also see Kisses of Death in the Graduate School Application Process, page 2, "Harmful letters of recommendation," subsection "Inappropriate sources."