It's not difficult for someone to set up a fake FB profile, purporting to be someone else.
It is possible that someone has viewed your friend's FB account and downloaded a few pictures and then set up a fake FB account, using these pictures to gain people's confidence and make it look kosher.
Ask your friend direct whether they have sent this friend request; or message the person on FB and ask them a secure question that only your friend will know the answer to ...... in the meantime, don't accept the friend request.