There wasn't collaboration other than plugging it into his code. It was not an oversight it was very clear he claimed to have written it all.You are correct though, partly my fault, why not mention it first myself, why wait for someone else to.
One final suggestion, if when bringing this up you come off as emotional as you do in your original post it won't be effective. Approach it as just a big misunderstanding. There is nothing wrong with ensuring correct attribution to work where its something big (which a library qualifies for).
