OP, consider this middle-of-the-road approach: Put some effort into training the teenager, but make it clear that it’s not just a “nice to know” kind of situation-- that you want her to remain a boarder but that can only happen if she lives up to your standards of horse care. A list is a good idea-- these are not difficult skills she needs to learn, she just needs to learn that tasks # 1, 2, 3 all need to be done on xyz schedule. Again, copy the parents in on this situation-- it’s not enough that they signed the initial paperwork, they have to know that they’ve been put on informal notice, and that formal notice will follow if improvement is not realized.
None of this has to be harsh or a big conflict. Just, “hey, we need to get better aligned on what self-care at this facility means. Because I enjoy having you and your daughter as boarders, I wanted to give you this heads up that there are some issues but we can hopefully get things back on track”
As long as you communicate all of this in a professional, simple, non-accusatory tone, then if they get mad at you, you can know that it’s entirely on them.
PS Guillherme is 100% right that boarding is not a money-making enterprise. Now that you have some history built up, it should be fairly easy to prepare a clear business plan, where you tally up the real costs and revenue. Be sure to factor in the commercial insurance policies (please make sure you have this!!!), increased farm maintenance, and assign a proper value to the time that you spend on managing the boarding biz. This last part is the area where most small biz owners fall short-- they view their time as free, but this is simply not true and can lead to resentment and burnout. Not to mention it hides a real risk that if you are incapacitated in any way, the business needs to have a plan to serve its clients, and that won’t be free. If hubby wants to view the farm as a business, none of this should be objectionable (not that you need his permission in any case) .