Your neck octave key is probably bent like someone else mentioned. This happens all the time to my students. It's very easy to fix although you have to know how to do it. I usually grab th top of the mecanism on the neck so it can't move and slightly bend the bottom of the mechanism either towards the neck or away from the neck depending on the problem. In your case you want to bend it away fro the neck. When you are pushing the octave key for the D it is hitting the top octave mechanism slightly and opening it. If you bensd it away slightly it will stop doing that. If you have no idea what I'm talking about I wouldn't mess with it. Just let your sax player friend do it.