If you pay the phone bill, one boundary you can do is cancel her phone. Be prepared for an angry outbust. Then calmly explain why is is not appropriate to use family money to enable her infidelity.
Last week she removed herself from our AT&T account to change carriers to Cricket Wireless. This was actually good for me because it removed the temptation of logging into see the records of who she was texting and calling. Of course, family money is still being used to pay for the new phone and carrier...
She also told me once a month ago or so when I caught her sexting and took her phone to read some of the messages that if I ever touched her phone again that she would break my fingers. Granted she was in a fit of rage from being exposed and violated, but the addiction and hold that device has on her in living out her fantasy is unreal. From what I read, the chemical sections that it can elicit are similar to a cocaine or heroin addiction. So, couple that with the dopamine and serotonin high of affairs and this condition is a monster to combat.
Me:41 W:39 S:9 D:6 T:20 M:16 PA:8/22/18, BD:11/6/18 PA discovery & IHS:12/3/18, W moves:4/2/19 R’ville:9/27/19, I give D docs:3/1/20 W home:4/5/20 (due to CV-19), W NC w/OM:4/13/20 6/1/20