Article RE: Begin To Jump & Xray?

Hi, I have a client with a TB that is just turning 3. He is off the track but has been in full training for almost 6 months now and is doing AMAZING. We have begun to teach him to jump small jumps, however the owners want to push him to go faster... I want to find some information to help them understand my two points:

1. While jumping a 3 year old horse is "ok" you have to be careful to go slow and not skip steps in the training or overwork their bones and muscles. Jumping more than once a week is unnecessary as well as is jumping over 2'6".

2. The knees should be Xrayed prior to jumping on a regular basis OR jumping higher than about 2'.

I have tried to explain all of this to them but their "friend" that they bought the horse from keeps telling them that I'm just trying to waste their time and money... I told them I absolutly will not jump the horse higher than 18" until they get the Xrays done and I am seriously considering telling them to just take their horse elsewhere if they aren't going to trust me - but I wanted to try to find some decent information that I can show them before I do that...

Thanks!