What would you do?

He blames his brother for using his phone for talking to this girl from last week Wednesday. But why would his brother use my boyfriends phone to talk to girls instead of using his own phone? The girl claims that my boyfriend is her ex and that my boyfriend been telling her he misses her and she treats him better than me. And I sent a pocture of my boyfriend to her and she said "yes that's him." She won't tell me how they met, how long have they been dating (WHICH SHE SAYS SHE DON'T KNOW), and what have my friend been saying to her since then. My boyfriend keeps saying he doesn't know who she is and why she claiming my boyfriend is her ex. But what would you do? That's not me sticking the middle finger though.