I include a disclaimer when delivering native files:
.... makes no promise, provides no guarantee, and offers no support, for the continued use, alteration, or editing of native files after files have been delivered. It is the sole responsibility of the client to ensure they have proper software, hardware, and expertise in order to utilize any native files should an agreement for delivery be reached.
I then merely refer back to it if asked to "train" someone on my files.
"Sorry, I don't have the time to dedicate to training others in the use of software. Adobe offers online tutorials and there are additional places such as Lynda.com which will offer courses on [application]."
To me, it doesn't matter if they are my best client or my worst. I just don't have the time teaching requires because I don't want to be a full time instructor.
There's nothing wrong with teaching them (for a fee) if that's what you want to do.
However, depending upon your business model, you may just want to decline any instruction at all. Realize that it will take a lot of your time. Much more than you can estimate. Time you may not have to spare if you have other clients and other projects.
In addition, they will most likely be in a hurry to learn anything and everything in order to make the edits they want to make. So they will not be patient learners.
And... you have no idea what type of students they will be. Some students pick up things rather quickly, others require you to repeat the exact same thing several times before the concept sinks in.
Teaching is great... but it is its own animal. Requiring you to feed it and babysit when necessary. If you want your career to move more in that direction, then go for it! But, if you want your focus to stay on design, then stay on design and decline teaching.
I love sharing what I know, but I hate teaching. I've done it, in a classroom environment and one-on-one. For me, they both have way more "cons" than "pros". But then... it's probably personality based for me. I'm not great at biting my tongue when I've told someone something 15 times and they haven't heard a word I've said.
(Quite honestly, if you decide to teach, I'd consider more in the range of several hundred dollars per hour. One-on-one training is much different than any general course, and the pricing should reflect that. If they just want to learn, there are online courses for probably close to what you'd charge your normal hourly rate. You should make it worth your time if you intend to teach.)
Another option may be on-site workshops... you travel to their business... set up a conference room somewhere at their location and then give 3-5 days of dedicated instruction. They pay for your travel, lodging, per diem, and instruction in one lump sum. The benefit here is that when training is done, it's done. If they want more... they pay more.
So you could offer a 3 day workshop for up to 10 students at their business for $xx,xxx.xx. You work out what it would cost you to travel and lodge for that time, $150 for food/expenses ($50/day), a rental car for 3 days, and then 3 - 8 hour days of your time.
If I teach, this is how I prefer to do it. You do have to work up a syllabus and handouts (3-ring binder as a "manual" or "cheat sheet" is fine) for the training, but it's better than trying to do the email or phone call exchange to train anyone. It does take far more planning (so include planning time in the cost), but it is often the most headache-free way to train when asked.