there's no way they could have had any contact of that sort unless she's been touchering her bf's ex gf.
1) he goes out with this woman, they break up
2) he goes out with katiej, his ex fucks katiej's bro
so yes, katiej's bf has had relations with the woman who fucked her bro, but that was AFTER they broke up, at the same time katiej is now dating the man. so unless katiej's bf is cheating on her with his ex while his ex is fucking katiej's bro, there is no connection.