Have you ever been betrayed by a friend? It happened to me in the past and I must say, it’s pretty awful! Unfortunately, it’s something common and there are several ways to refer to this kind of people:

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Example sentences from the web:

• “What do you think about frenemies?”
  “I say, keep your friends close, and your frenemies closer.”
• Zack is John’s frenemy. They get a long in the office but both of them work on internal competing teams.
• Gwyneth had a frenemy once and was troubled at the joy she felt when thisperson suffered a terrible public humiliation.

“Frenemy” on the Web: How to Deal With Frenemies

P.S.: I’m sure you got it, but if you didn’t … This term is a combination of the words “friend” and “enemy” ;-).