Easy - he has crossed the supply pipes to the shower. Cold supply is on the right, hot on the left.
This is a common mistake and i have done this in my apprentiship years once before - a good bollocking off the boss made sure i never forget! However some shower instructions arent always the clearest if its one of those built in behind the tiles so dont be too hard on him!
The way these things are sometimes piped, and in your case from your description, the shower is tapped off the supplies for the bath. Sometimes the bath taps are the wrong way round from the very beginning. Should be H&C not C&H. Cold is on the right always - the reason being most people are right handed and if you needed cold water quick its instinctive to put out your leading hand!
If the bath has been done a straight swap and the taps have remained the same - its easy just to bring two pipes off the bath supplies straight up the wall to the shower. This is fine, but if the taps are the wrong way around then this is just being lazy and he should have dealt with putting the bath supplies the right way in the first place. You try and cheat a problem it usually comes back to bite you twicefold in plumbing.
You may be able to turn the shower unit body (the internals) around to suit the reversed supplies (some manufacturers allow for reversed suplies and make their showers reversable but not all) without the control handle being upside down.
If not your installer will just have to bite the bullet and switch the supplies. He may be able to do this from underneath the bath if hes lucky and allowed plenty of access space. If you have had your bath tiled in then im afraid its 'Hammer Time'!
If it is supplied from the loft to your shower and the supplies drop down, and the bath supplies come up- then its a piece of cake. Just swap them over in the loft.
Oh and the shower supplies should be balanced. Both hot and cold should be either off the combi, or multipoint water heater, or cold main and unvented cylinder (with the correct flow restrictors fitted and if a balancing shower and not a mixing/thermostatic shower non return valve(s) should be fitted to stop backflow), or cold off the cold header tank and hot off the cylinder preferably on dedicated supplies. If its a cold main for the cold, and a hot off the cylinder then there is a very good chance it wont ever work right, reversed supplies or not.
If its piped up off the bath supplies, and your bath is cylinder hot and cold main it could be dangerous. If you get in the shower and get it balanced just right, and your cylinder has the usual 60degC water going thru it mixed down with the cold, and someone else in the house flushes the loo or runs a cold tap... what will happen? The quantity of cold will lower and you run the very real risk of being scalded.
Make sure your shower is a thermostatic shower. Make sure it has balanced supplies. Make sure the supplies arent crossed. It cant have hot tank and cold main, its a burn risk.
Im sure its a simple mistake and he has just crossed the supples and all will be fine - just seen to many bad instals in my time and too many accidents by unqualified/lazy installers!
Plumbing and Heating engineer for 12 years and makes you a little cynical - there are a lot of rogues out there!
Oh and dont take any crap - it wont be fine as it is, cold first, hot second, thats the rule. Its an illegal install and thats the way a small claims court would see it!
Good luck and ask if you need anything else or just contact.