Starline, I don't think you're an awful BO at all. Here's how I handle emergency things that come up. If you (the owner) caused it with neglect and ESPECIALLY if you want to be a cheap B*stard about it, I charge out the wazoo to make sure it hurts you plenty.
For instance when I lived in Tucson I had a boarder who took his mare and gelding out and rode them until they were dripping and would come back home and turn them loose and not cool them out, rinse them off, make sure they drank, NOTHING. This is NOT the way I approve of taking care of horses and I let him know about it more than once. Every time this man rode, I was standing there to be his Juan and take his horses off to walk them out and rinse them down and make sure they got a good drink as they cooled out. He didn't pay a DIME for that service on top of his board, never even said thank you, but I felt for his horses. Well, his mare was a sweet little thing and older, I think 21 or so. One day I noticed she was off her feed and checked her water and she wasn't drinking either. I called him, got permission to call the vet and called the most expensive vet in the area. He was also the best, least nonsense about him vet in the area. The owner showed up talked about putting down a perfectly good mare that HE dehydrated **** near to death (it was August and 115 frickin degrees out) so that he wouldn't have to pay to hospitalize her. I talked to the vet and got him to give me the IV solutions and meds and hook me up with everything she needed. I then set myself up a cot and slept in front of that mare's stall and changed her IV's, gave her the electrolytes and meds to rehydrate her and get her going again for 1 solid week. She'd probably have only needed to be in an air conditioned vet hospital for a couple of days but out in my mare motel, no AC, no misters etc, she needed constant attention and monitoring. TRUST me, that man got a WHOPPER of a bill at the end of the month for the extensive nursing care that little mare needed. And the vet cut him no slack either. She came back and was ok but I really watched her close after that. By the way, he never visited her once during the illness, only after she was ridable again. Yep, I made THAT bill HURT.
Now, someone who makes a mistake or out of not knowing or their horse just gets sick, I don't charge for the initial care. If I haul the horse to the vet, or if I have to change bandages or give meds a few times/day for a while if the owner cannot get out to do it or doesn't feel competent, then I charge. If I had a colicky horse and the owner said, call the vet and call me again if it's that bad and they didn't want to respond, it's THEIR horse and THEIR responsibility. At that point, I go on the clock and I'm not cheap.
If the horse gets hurt on my fencing, then I look at my fencing and try to figure out what I can change so it doesn't happen again, but I don't charge to hold the horse for the vet. The first time the horse nailed me in the nose with his head would be the last day I tried to deworm him with a paste tube, I'd put the wormer in his food and eat it or not or deworm him yourself. My personal horses are not difficult like that and if I'm not being paid to train as well as deworm, it's your (the owner's) problem if he's a problem child.
If he's difficult during lessons and your assistant runs the risk of being injured by him, I'd find another horse for my lesson program and the owner could pay full board. I guarantee you, if your assitant hurt that horse Pookie would become a Million Dollar Baby, and if Pookie hurt your assistant, it would be your liability not the owner's. Just my years of sometimes painful experience talking here.
As far as walking a horse with colic, I don't. My vet says that unless they are literally thrashing, let them lay down and be quiet, it's better for them. If thrashing then he says to tranq them if possible, if not, load 'em up in the trailer and get them to the hospital. (I'm only 10 mins from the vet hospital, it's cheaper to pay me to haul them there for $50 round trip than the farm call from the vet $65.)