We used to canter on the roads when we where hunting or went for long hacks and trotted the horses up steep hills to get them fit for the hunting season (o.k. years ago)!
You are lucky to have found someone to help you - just explain why one should not canter or trot on the roads.
I would not be pleased. I have cantered on the road gently when hunting - as granny used to say, better than a hammering trot and she had been hunting for over 70 years. That said, it was my horse, sound as they come and more importantly it was my horse and my decision.
I would be even more angry with regard to the mares - unless I am mistaken he rode without permission or any regard to their condition.
I would have thought anyone with any ounce of common sense would have know not to do this with pregnant mares.
Personally I would not have them ride my horse again until I was 100% convinced that they knew what they were doing, and to be honest would look for someone else to exercise them. If he was that unknowledgable I would not have let him ride mine in the first place without supervision.
Regardless of whether people agree with cantering on the roads, this horse doesn't do any fast work on the roads, including much trot, from what you say. A horse that does a lot of trot/hunting work will have the dense bone required to cope with such concussion, but I bet YOUR horse had bloody sore shins after that - I know I do if I jog on the roads now, but if I built it up slowly I wouldn't have a problem. I think if I was the YO I wouldn't allow him to keep a horse at the yard. I wouldn't trust myself not to chase him with a cattle prod or summat!
I would not let him ride them again, unless you are really desperate for the help, in which case make sure you supervise him. I had a similar thing, was struggling to get my daughters horse fit (she is at uni) after he'd had 3 months off with a virus. My ex's partner (I know weird situation ) offered to help she apparently had loads of experience and saw her ride him and she seemed to handle him well. Was a bit concerned when he came back a bit hot, but hasn't been clipped and was woolly so put it down to that. Then 2 weeks ago (he is now fully fit) I was at home when she took him out and watched out of the window. She started trotting within about 20 paces of home and trotted for a mile at least , after that she went out of sight. When she came back I told her I didn't like that much trotting and the reply I got was, he's been hunted for years his legs will be used to it! yes he did hunt for 5 years, which is one reason why we treat him a bit more carefully now. I wasn't sure what to do but think she has taken offence and hasn't ridden him again, which in a way am glad about but does mean his work level has dropped again. Sorry to hijack your thread, this has been bugging me since it happened, was I over reacting?
I'm not even cantering on grass at the moment as the ground round here is rock hard. It just proves how ignorant he is if he doesn't realise what he is doing wrong!
I'd go mad! I've only ever cantered on the road once and that was when a car drove into the back of my horse and she took off! I don't think I've ever even trotted my boy on the road yet, and would certainly never intentionally canter! As for the pregnant mares...that boy would not have been physically able to get on a horse again after I'd finished expressing my opinion!