I misspoke earlier and need to make a correction to what I wrote.
I said the musical group is (potentially) responsible for infringing on Matt's poetry and they are the ones who should be targeted with the cease and desist letter - and this is still true. However, it is also true that the record company can share in the liability for distributing copyright-infringed material, this even if they were unaware the material infringed.
Whatever the case, Matt needs to consult with an attorney for a review of all of the facts (including whether the band/recording company has registered a copyright in the lyrics, making them the presumed owners, and the dates involved in the creations of both the poem and the song).
I apologize for the error.
Zig's idea is good.