Sure - he crossed the purple wire with the blue wire which causes the white
wire to feed back voltage through the black wire and the net result is that
the red wire never sees the difference in potential that it expects, so the
grey wire fails to go active, and your brake pedal wire monitor gets all
screwed up.

Now really - you don't say a thing about what the original problem was,
don't say any more about what your friend did to "fix" it, and you expect a
group of people who don't even know what model Hyundai you own, to diagnose
your problem.  Sure - lots of people here can help - with some information.
Absent that, we're all going to be poking in the dark.

Why did he attempt to repair the tail lamp wiring?  What was the original
problem?

Other than the dash lights problem, do the tail lights seem to work
properly?  Brake, tail and directional?

Where was this friend monkeying about on your car?

Are you opposed to simply taking your car to a qualified mechanic?  This may
or may not be a requirement for you, but based on your original question,
I'm guessing it's going to prove to be likely.