Halsey gave her pals a sweet treat at her Halloween party on Friday night -- by bringing out Avril Lavigne for a surprise duet on Avril's "Girlfriend." Avril, who took a break from her first tour in five years to attend the costume party, dressed up asMadonna-- because the theme was "Almost Famous: Celebrating the Rock Stars of the '60s, '70s, '80s and '90s. Halsey came as Marilyn Manson, an outfit she also showcased on Instagram.