That´s chap.6 in Sugars (Yamamori Mika)! Btw chap.8 is a direct spin-off of that story^^
http://www.mangago.me/read-manga/sugars_yamamori_mika/mh/157274/Vol1_Ch6/pg-1/
The girls does work as a dj but she doesnt really steal his girlfriend, but I remember another manga where there was a girl who dressed as a guy to steal her childhood friend's girlfriend b/c she liked the childhood friend but I'm not sure if your talking about that manga or not
This isn't a yaoi manga. This girl looks a lot like a guy and is a dj at a club (or something like that) and dresses up like a guy and steals her best friend's, who's a guy, girlfriends because she likes him... I read it one and loved it but then I couldn't find it again! Please help me!(T-T)