Ummm actually you said you two have been in a rough patch for 4 months, so it didn't just start with her. Sounds like it's escalating things though. Don't let this distract. I don't think people get to the "brink of divorce" just from fighting over texts for a couple days. To save a marriage, each needs to own what they've brought to the table.

You trust him or you don't. Either way, stay off his phone. It's an invasion of privacy and not conducive to good relations.