Here's what comes to mind:

The term "friend" is rediculously ambiguous, particularly if you're looking to cement a lasting relationship. Rather than blow up at her, simply talk to her in a honest and calm manner - let her know how you feel about her "flirting" with others. Then let her respond.
There's no reason for you to get hurt on account of her behavior. If she wants out of the relationship to go flirt, then let her go IMHO.

My 2.3¥