If you have a friend that now is asking you to friend them again ... beware ... that may not be the person you think they are. Look at their profile ... Does it list their town they are in ... Birthday ... etc ... Are they messaging you and never have in the past?
All signs they are not who you think they are. Do Not add them as a friend and if you have, you will see you them twice in your Facebook friends ... Unfriendly the bad one ... when the person changes their password it will ask them if they want to sign out everywhere and they say YES ... they will not be able to get back in to become them.