Thank you for your input, thus far.
One of the complications is that both my friend and I are involved in a degree of intertextual writing, with the main difference being that I am scrupulous about noting the texts I quote from, use in centos (a poem made up from quotations from other poems), terminals (a poem that uses the same final words as another poem), glosses (a poem that comments on another poem) and so on. This is regardless of the notion that all texts refer to other texts by their very nature (that is, apart from quotation).
In this he has attempted to justify his actions, invoking that tradition of intertextuality as a poetic strategy, without admitting to plagiarism. I have, after reading the above comments, emailed and said that I recommend he admit to plagiarism, and not try to justify himself, as the latter always makes him look worse in the eyes of his critics and enemies (the poetry world can be quite factional and prone to backbiting).
Anyway, I hope it works out. I'm being honest with him, straightforward yet as tactful as is possible with him.
Thank you for your input, and feel free to add more as you see fit.