Those “private profile viewer” sites are basically all scams—they just can’t bypass Facebook’s privacy settings from the outside. Your best bet is installing a monitoring app directly on their phone, which captures FB activity from the inside; mSpy does this well, though it requires physical access to install and won’t work if you can’t get the device in hand. TL;DR: if you want an external “viewer,” you’re out of luck—they don’t exist; if you want actual data, use mSpy on the device itself.