A colleague to me has another opinion than me about the amount of horizontal space before an ellipsis.

My opinion:
In my mind, an ellipsis can be used when a word is interrupted (a) - or when a sentence is interrupted (b):

(a) “What the f...”
(b) “Well, I don’t know ...”

I think it is logical to have no space in (a) and a space in (b). The logic being, that the first point of the ellipsis is placed precisely where the first left-out character should have been.

My colleagues opinion:
In example (b) she would reduce the horizontal space before the ellipsis, much as you would e.g. a semi-colon.
Have you ever seen that done? To me, it makes it harder to distinguish between the a- and the b-type.