Be assertive and set a firm deadline. I have ADD, and I'm terrible about deadlines. If I miss a deadline, I need a new deadline. A past deadline is like no deadline at all. Waffling does not motivate me. If someone was to ask me when they could expect something, I would not know how to answer. Because a) I have no sense of how long it takes me to do something, and b) there is a lot of shame around missing a deadline. I wish more people would tell me that I am inconveniencing them--yes, I should just "know" that, but if I hear it from the person I take it much more seriously. That's just me. Who knows why your person is procrastinating, but in any case, be firm.
I just got "Boundaries: Face to Face (How to have that difficult conversation you've been avoiding)" by Cloud and Townsend out of the library and it's very good, so I would reccomend that. It's full of examples and different types of conversations, how to prep for them, etc. I'm looking forward to reading the rest of the series. The books have a Christian bent, but I'm not a Christian and I did not find that part of it objectionable at all. If you are a Christian or open to spiritual teachings, it just enhances the authors' message. I hope that helps!