As far as we know, the EVE client is written in C++ and Python. C++ handles all the low-level stuff like 3D rendering, network communication, input handling and such. (Stackless) Python is used for everything else, which includes the user interface and even the management of graphic resources (you can see this in the debug window by pressing ctrl+alt+shift+m). This technology mix is quite unique, so I’d suggest CCP implemented the EVE GUI in-house, not using a (open source) widget library. I’ve been playing EVE for many years now, and the GUI has evolved a lot since the days I first tried EVE (around 2005). This also strongly suggests that the code was written by CCP.
There are two articles mentioning technology used in the client: