I believe my daughter’s iPhone is cloned.

I believe that my ex has cloned my daughters IPhone. I think he did this to another phone she had a few years ago too. When I go to the text log on my Verizon account online on the laptop, the texts between her and I don’t show up, and I get texts from her when she’s not with me that sound like they’re coming from her dad. She goes thru data like crazy while she’s at my place even tho she’s connected to my wifi.
How do I stop this?