I've been in a similar situation before, and its complete s***. So....here's my advice.
In a court of law, assuming this was some criminal thing (it isn't), evidence obtained illegally (ie without a warrant) doesnt stand up as proper evidence. Based on this, i'd confront her about it but not mention you've seen her texts (without her permission). Just mention that you've seen she has gotten texts from him when they flash up on her locked phone (im assuming youve seen that, right?) and you really arent comfortable with it, and see what she says.