I'm confused you are in love with a girl and are wondering if it's ok to be friends with her best friend or boy friend? (BF means both and in this context it's a bit confusing)

Either way I can't see anything wrong with it as long as you actually like the person or can stomach building some common ground between the two of you to build a friendship on.

For instance while my husband isn't best friends with my boyfriend they are working on building a relationship that is more than just friendly. It isn't easy sometmes but it's usually worth the effort.

Now if the persona is just not a good person or hurts your loved one then you are under no obligation to pretend to enjoy their company, though it is usually better to be polite and distant. If the friendship predates your realtionship with her then you should tread carefully.