I'm disappointed: The lyricist of my music copied his words from other source!

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I joined a song writing contest in our school 10 years ago. I had already my lyrics and music. But somebody approached me and asked me to let his lyrics be included in my composition. Because I considered him as a friend and without any doubt, I agreed to let him be the lyricist and I was responsible of the tune/ music. I discarded my original lyrics in exchange to his. Fortunately our entry won the first place. And it became the hymn of the school. Recently, it was found out that some of the lyrics were copied from a hymn of other school in a very far place. I was really disappointed because I trusted the person. I could not believe what he did. It was a great mistake and I regretted why I say yes to his copied lyrics. If I did not allow him to be my tandem in that school hymn, this would never happen. The school president passed a memo to stop using the school hymn.