Here's 5 ways to know if its fake or not
1. Check friends list, if more or less than the video showed you, might be a fake
2. Check badges, if more, might be a fake
3. Check avatar, if changed, might be a fake
4. Use an extension to figure out when they were last online, if they were online after the video was posted, might be a fake
5. Check if they have 0 friends, if they have no friends, then how would the tiktoker know? If they have no friends, might be a fake.