Chief, that looks like it may be fairly simple to fix if you are living there permanently.
Its fixable if you are renting, but you got to do a neater job.....
It looks like you have a black power wire coming into the switch from the panel, along with the white neutral.
Then the white neutral and the two output neutrals are all tied together.
Those can be left alone.
Then, take the one hot black wire loose, and split it up to two wires.
Leave one going to the dimmer like it is now, and take the second and hook it to another switch which is rated high enough for the fan.
Take the black output wire going to the fan loose from the dimmer and put it on the other side of the new switch.
Now comes the tricky part.
You can buy "add on" boxes that will hook to the other side of the stud, or even snap into a hole in the sheetrock.
Mount one of those, somehow run the two wires hooked to the new switch over to it, and mount the switch in that new box.
How you get those wires over, depends on how you mounted that new box.
I have seen a hole cut in the side of the old box, plus a couple of smaller holes. Then the new box with the same holes cut in it was slid into a cutout in the sheetrock on the nonstud side of the old box, and the two were bolted together with the wires running through the bigger hole.
I'm not saying do it that way, that would not be code, and therefore wrong. Just saying I have seen it done that way............