Chris' father keeps getting a text message from Facebook, and it appears to be legit, but his dad doesn't even have a Facebook account. Leo says that somebody may have their phone numbers mixed up and keeps adding Chris' dad's phone number to their account. If his dad has a new cellphone number, the number may have been used by someone on Facebook and that's why he's getting it.
He could try replying to the text with 'STOP' or 'OFF' and that might stop the messages. At least until the next time they add the number. The Chatroom says he can call Verizon and request disabling of "Short codes." That could get rid of this too.
Facebook has something called "login approvals" which prevent users with stolen passwords to log in from another device. That may be why Chris' dad keeps getting texts. He could try creating a Facebook account, add that number and approve it, and that may stop it. Leo recommends using login approvals in Facebook anyway, since it secures the account better.