Complex sentence: They then visit Lacey, one of Margo's best friends who Margo feels has had never been a good friend and has often ridiculed her physical appearance.
Split into simpler sentences: They then visit Lacey, one of Margo's best friends. Margo has always felt that Lacey had never been a good friend to her because she had often ridiculed her and made backhand comments about her physical appearance.