Dealing with your fiancé's ex wife?

Yesterday my fiancé and I had a pretty big fight. I discovered that his ex wife has been texting him and he's been responding to them. This isn't the first time it's happened and even though he's asked her to stop, she continues. I feel like she's constantly butting into our relationship by asking if we wanna go do something with her and her new boyfriend. I feel like it's a trick so she can attempt to steal him back. Don't get me wrong, she's nice and under different circumstances we could've probably been pretty good friends but I just feel terribly awkward whenever she's around or mentioned. There's truthfully no reason for her to keep in contact with us, since there are no children in the picture. I need a way for me or my fiance to politely tell her how we feel. I heard last night that apparently she's been suicidal in the past and actually had to get psychiatric help. I don't wanna say something that pushes her over the edge, just something to get the point across. Help please! Nothing too harsh.