The only time you can put the before a person's name is when you make the person into an adjective.You do not want to include a definite article ("the") in your sentence. It should be: Who is Cecilia's friend?
In fact, you would rarely, if ever, use an article (definite or indefinite) in front of a person's name. Adding the definite article would be the same as saying, "Quién es la amiga de
laCecilia?" in Spanish.