Hello novazela,
If we're understanding you right Picture in Picture is intermittently not woking on your girlfriend's iPhone 8 Plus when on FaceTime video calls. We know how convenient this feature is for multitasking, and we'd like to see if we can help get it working as expected on her device.
Since it sounds like this is a software-related issue, we recommend reinstalling the iOS and then checking to see if the issue persists. Prior to doing this have her make a backup of here iPhone: How to back up your iPhone, iPad, and iPod touch. Once the backup is created, restore the iPhone to factory settings using these steps: Restore your iPhone, iPad, or iPod to factory settings.
After the restore is complete you will be walked through set-up steps and will be given the option to restore all of her data from a backup. Skip restoring from backup and set the iPhone up as if it's a new device instead. This will allow you to test out the iPhone with a fresh iOS install with nothing on it.
After testing out Picture in Picture, she can then restore all of her data from her backup using these steps: Restore your iPhone, iPad, or iPod touch from a backup. Test it out once more after restoring from backup.
It's possible the iOS reinstall itself may resolve the issue completely. However, it's also possible the issue is put back on the iPhone after restoring from the backup. If this is the case, restoring once more more to factory may be required and then loading all of her data back without using a backup.
Thanks for using Apple Support Communities. Respond back if you need continued support with this issue.
Take care!
