She can't be friends with her ex - and be your gf - if she has an argument with you, who is she likely to turn to - in many cases it's her ex. For some bizarre reason, many people have convinced themselves that being intimate with an ex doesn't count as cheating (when they are in an active intimate relationship with someone else). If you can't convince her, logically, she's still involved with him - end it, recover, find a girl/woman that understands how intimacy works between partners that consider themselves to be a "couple" (exclusive to each other).
Your insecurity is making you jealous. Either you trust her or you don't. You can't go around telling people who they can be friends with. You need to back off and say nothing more about it. She'll get sick of playing silly games when she sees she can't get you upset.
Why are you worried? She is with you right? Man up and try not to control her. (I know it is hard) but everyone has friends that others don't like but it takes a man to let her have free will. She will respect you more or go the other route, if that is the case she did you a favor. Love is free will
You should probably break up with her cause you're obviously jealous and insecure and the only way you can find to fix it is to control her and "make" her not be friends with him. After reading the other questions it seems like you're only referring to her being friends over Facebook anyhow which makes you're paranoia only seem worse.