It is perfectly okay to set boundaries and to choose who you want to have close relationships with. If you didn't want to be best friends, even though she really wanted to, that's not a disorder, it's having and expressing your preferences. Sometimes our choices cause someone a bit of discomfort. Did her insistance on you being her best friend cause you some discomfort? Does that mean that she has a personality disorder?

It really comes down to broad, long-standing patterns. When those patterns have a significant impact on your life, or someone else's, over a long period of time, then it might be related to a personality disorder.

BTW, one thing that is often said about personality disorders is that the symptoms of the personality disorder often do bother other people more than the person who has the symptoms. But then, personality disorders are often associated with other disorders and symptoms which do create problems and bother the person who has the disorder.