Look, I get the concern - been there with weird phone behavior myself. But just heads up, going the “without them knowing” route usually backfires spectacularly when they find out. And they will find out.
That said, eyeZy (which, you know, you’re on their forum) does browser tracking. Works on both platforms if you can get it installed. The “without knowing” part though… that’s the tricky bit, legally and relationship-wise.
Have you tried just… talking to her first? I know, revolutionary concept. Sometimes the “acting weird” is about stuff that has nothing to do with sketchy browsing. Could save you a subscription fee and a trust meltdown.
What kind of “weird” are we talking about here?