Summarize the following:
When looking at a flirting situation, it often takes two to tango. The other girl may initiate the flirting with your boyfriend, but your boyfriend may play a part, too. Try to look closely at their interaction to see if they're both flirting or if it's really one-sided.  The point of analyzing the situation is to see how to better deal with it. Once you figure out who exactly is involved with the flirtation, you can decide whether to deal with just the other girl, your boyfriend, or both. It can also help you decide when to time any conversations you have. For instance, if she's leaning in close to him, batting her eyelashes, or puffing out her chest, she's flirting with him, especially if she engages in flirty banter. If he leans in close to her and encourages the banter, then he's flirting back. If he doesn't engage in the behavior with her, he's trying to shut down the flirting. If your boyfriend's not engaging, focus on dealing with her in the moment. You can find tips on doing so in another section of this article. You don't have to get mean, but you can deal with the flirting in subtle but meaningful ways that will help get the point across.  You can simply raise your eyebrows at her to let her you know what she's doing. You may also choose not to deal with it. If you're out in public, you may just want to move on. If your boyfriend is either starting or engaging in the behavior, wait to deal with him until you can talk to him in private. That way, you don't start a serious discussion in public, and you can do it when you aren't so agitated.
Step back and look at the situation objectively. Focus on her if she's the problem. Wait to deal with him.