Yes making a stand is one thing. But their are proper ways to confront and not react.

Like Mr. B says. You don't have to be over the top. My thoughts are along the lines of this:

"are you txting men from your cell phone" she can say yes or no. If no, I can say well I know for a fact you are contacting Mr. X.

her: so what etc (25 always says to be prepared for this type of response)

"I find it extremely disrespectful, is there something we can talk about here?"

her: no to bad

me: "ok, that is your choice, but I am no longer going to pay for your cell phone bill if you continue with this behavior"

her "f off, azzhole" (boy do i know my w)

I guess my point it would be nice to role play with some ideas before I actually committ to it. Actually practice the confrontation in the mirror if I have too.