I tried everything but again I'm hitting a wall here. I have one frame split in the middle with a vertical line giving me a window left and right. Now I have Ibuffer open in the left window and I just want to press a key and the selected buffer should open in the right window WITHOUT opening another window. Whatever I do I either get Ibuffer to open the selected buffer in it's own buffer (pressing Return for example) or getting it to open it new window or frame. It seems impossible for it to open it in the already opened window on the right. I'm not a coder so I can't help myself using some lisp.
This needs more investigation on negative side-effects, but setting
display-buffer-use-some-window seem to work. Found hint to it on
Using the mentioned variable with the hook from glucas mentioned below seems to work and limits possible side effects to ibuffer. It's a workaround, but better than nothing.