I fully understand this. I've spoken with mom and we've spoken together with our daughter and told her not to be texting at school at all. We are in agreement on this. I'm well aware that our rules are different and what happens there I have no control over, nor do I try to. I try to keep the mom informed about what our daughter does when I notice it, and I've told her, if she doesn't care, then to tell me and I won't tell her.
I also make sure that if she gets in trouble with her mom that I don't enforce her punishments here without a good reason, and I try not to have my punishments in effect at her mom's house either unless I think there's a good reason that's not household specific. (i.e. bad grades, yes, not doing her chores here... not her mother's problem)
She doesn't deny it, but she doesn't really 'talk' either. You ask her questions and she just stands there mute not wanting to answer.