Maybe, but don’t think of it as a good enough reason to look through her phone. Think further ahead than that.
If you take those first steps and violate her privacy, then you have to be willing to follow through with a potentially relationship ending intervention.
Are you prepared to face the consequences? Because whatever turns up on her phone, this shit isn’t gonna end well. It’s either gonna confirm what you already know to be true, or it’s evidence that neither of you can trust one another in the first place.
You’re fucked either way, so you know, do what you gotta do to keep her safe right up until you’re done, and once you’re done, stay fucking done.