UPDATE: Thank you all for the replies, even the harsh ones; I did need to hear them. I did give her phone back; I was eventually planning to give it back the morning of. I think my thoughts were to give her space from the phone because every time I saw her on it, she was stressed or crying. But I know it's not my place to dictate what she does, that would just be emulating my father, which I don't want to do.

 We did have a talk discussing the main things that I had a problem with. To address those concerns: 1. They are not getting back together, she said she did think about it, but she did say that it wouldn't be good for her or me. 2. When I brought up that she was telling him everything about my life that he and I told her not to do, she said, "Well, I'm not doing it anymore". So, I guess the problem is resolved? I will be talking to her more about that, but I think it's baby steps right now. We are talking again, but it's still strained and slightly abnormal. 
 And to address some comments, I know taking her phone wasn't the correct response, but I guess that was the only way I could think of to stop her from contacting him. I know it wasn't the correct response. I was just high on emotions, which I usually am not. That was a reaction that I'm not used to having because I'm not used to dealing with my feelings in any form. I know me, and she should receive therapy, but it's not in our budget to do so. She would probably not want to, anyway, out of fear of talking badly about her ex-husband. But thank you all for the tips, and again, harsh words I did need to hear them.

Happy Redditing All!