Good for you for sharing and getting some frustration off your chest. That's one of the reasons we are all here to support each other so we can move on before we go back to our horses. Also good to see you have maintained a sense of humour
First of all, I think we have all been there worrying that our horse is sore / sick etc and that is because we care and because we want to listen to our horses. That is a good trait to have. However, it can have knock-on effects. Essentially, it can create doubt as to what we are asking for and that diminishes our leadership. It is no coincidence that Leadership is the title of the first module in the programme. Without it, we can have little positive influence on our horses as they are either completely indifferent to us or, worse, nervous and unsafe with us.
So what to do if we get a sense that something is not right? Like you say - when was the last time you saw a chiro? Do you throw tantrums when somebody asks something of you? Do you scream and shout and throw yourself on the floor? Presumably not.(Perhaps you do! They say our horses mirror us!) The point I'm making is that even if he is sore, wouldn't it be good if he had the emotional fitness to try anyway, just a little; to think his way through the situation an not just react? If you keep that in mind next time there is a modicum of doubt, it might just help you to maintain that leadership. To be fair, it sounds as if you know all that but when his first card didn't win the trick, he played the joker and got you!
If he tries to roll on the end of the rope again, stop him! Or get him up as quickly as you can and give him something to do. Rolling is absolutely not an option. Imagine if you were riding him!
So, what to do next? Well luckily, every day is a new day and we get to start again Every day we have to work with the horse we have today. So move on.. don't go out there worrying if it's going to happen again. If you think it's behavioural and not physical, keep him busy. Make sure you are being interesting (these wonderful creatures are way quicker than we are and can get bored very easily - this is a constant challenge for me with my mare). Film something to submit for feedback - it may be that your coach will see something and can help you - perhaps you are giving your horse mixed messages with your position. If you get stuck, consider getting live help from a QS professional (Instructor or Horse Trainer) sooner rather than later.
Tell your friend that the good news is that the programme is designed to help us through all this and the further you progress the less likely this behaviour becomes. So, he/she should keep at it and get through level 1 asap.