What's the anime about a redhead whose friend uses magic?
The anime you're talking about is Zetsuen no Tempest (or Blast of Tempest, as Crunchyroll translates it). 

It begins with TAKIGAWA Yoshino meeting his long-time friend FUWA Mahiro, who has gained magic powers from KUSARIBE Hakaze (a magician stranded on an island in the middle of nowhere) as part of a deal in which Hakaze will help Mahiro figure out who killed his sister, FUWA Aika. Unbeknownst to Mahiro, Yoshino and Aika were lovers. Drama unfolds thereafter.