There's nothing wrong with this.
You're friends, so grammatical flexibility exists between you. It's fun to play with language, or maybe he was trying to be more efficient.
Grammatically it's no good and it shouldn't go in an essay, but don't be so hard on the guy! If you pay close enough attention, you'll see people make these kinds of mistakes in speech and online chatting all the time. In most cases, no one notices, and even if they do they forget about it soon enough. This is because we're capable of filling in the missing pieces in most situations and can understand 'broken up' or 'incomplete' transmissions for this reason.
It wastes a lot of time to be nit-picky about this with a fluent speaker. By definition he knows how to speak English perfectly, and therefore also knows what rules can and cannot be bent. He may say something like "why I didn't read" instead of "that's why I didn't read your thing" but he would never say "read I why didn't".
This is the distinction between "descriptive" and "prescriptive" grammars, essentially. English papers (and all academic/official writing) uses prescriptive grammar, in which the rules are created to be followed. Conversation and other informal language uses descriptive grammar, in which the rules are innate and up in your brain already, and it's the job of a linguist to figure them out!