Somone I spoke to had to sell her old trailer to finance their towing course and then the test, then had to get a loan to buy a new trailer again (well a second hand one)... she said it cost her over £300...didnt think it cost that much!
it costed me around 900 as its expensive where I live.. basically you are expected to drive to a very high standard and also show you are capable of towing safely etc.. you are best to get training as you are more likely to pass......
On the test you have to reverse the trailer into a bay putting the back of the trailer into a box. You have to unhitch drive forward and park by the side then hitch back up again. An emergency stop in the ministry controlled area and then about a 50 minute road drive.
Your friend really needs to get some tuition. I did mine last year and it was quite hard.
One really good tip would be not to tow the horse in the trailer on L plates at all. On the test you are expected to drive up to all of the speed limits - which is quite a bit faster than you go with a horse in.