Confiscation for the purposes of preservation of evidence would be okay, so long as it was all signed off and sealed properly.
Going into her Facebook account and pressing Delete or even adjusting the visibility? That's illegal tampering of evidence even if no "direct harm" was done by doing so.
Just pressing the buttons on a logged-in phone could be construed as such, as you then have NO idea what MIGHT have been in there before the tamperer got to the it - cop or bystander (and if it's a modern phone with encryption and flash, there may be no way to recover whatever you deleted, or even a way to tell that you deleted something).
It was deemed illegal to delete photos from the cameras, or even force photographers in public places to delete them by making a scary policeman do it themselves. This is exactly the same but happening immediately after a potential murder / attempted murder of a police officer (if there were genuinely at threat of death from the guy). It's a lot more serious.