If he has the ball joints on the height regulators strapped up that could well be affecting the height, unless you have fixed the problem, if not new height corrector links could be in order.

I haven't had any problems with them on the C5 although I did on the XMs, on those the height corrector could get stiff due to corrosion and lack of lubrication but I don't know if the C5 ones are the same.

The only way of altering the height as far as I know is by rotating the clamps on the anti roll bars, but as these have usually rusted solid they shouldn't have moved unless unless someone has altered them. There may be a small ammount of electronic adjustment but you will need to find someone with a Lexia to determine that.