I need some advice?

Im dating a wonderful man we have been dating for 1yr and 3 months TODAY. so the thing is his brother who im trying to help him find a girl. Better then he ex's who basicly used him for and what not. So back on the 4th i told him there is a gilr on line, through yahoo personals that he was useing that i knew and he liked her so i gave him her username to chat online and now they have been chating 4 2/3 week and talking (voice chat) as well. So the whole time he has been talking to me not this girl. Tonight i seen him chating with his ex's over at my my bf place (in person) who he told me him self that the only reason she wanted back with him is for and he told me he did want that. im trying to get him away from her by texting to his phone through yahoo messanger here so he can tell her bye/night what ever to just get away from her a**. what can i do with out letting my bf find out i was doing this and get his bro. away from his ex to find better?... If he goes back with her the