https://www.express.co.uk/sport/tennis/1662548/Rafael-Nadal-John-McEnroe-US-Open-press-conference
She's on a bad track right now. She won a slam without all of that intense weight training, and now it seems unattainable to repeat as the injuries pile up. After reading all the superb constructive comments about her situation, the idea of following the most successful and least injured pros is a great plan.
She might try Novak's old coach Marian Vajda. He could help create her recovery plan and limit the foolishness of heavy weight training and instead focus on flexibility, injury prevention, and resilience. This formula might offer a longer career path.
It often depended on the court space available and the number of students. I preferred to use live ball drills as soon as possible in conjunction with a graduated approach. Dead ball feeds were used for absolute beginners. I would use both for variation and also accomplish specific objectives.
There are many causes based on observation and personal experience. Aging is a common problem with all types of injuries and this is one of them. An aging body simply is more vulnerable and usually by the time you hit your 40s the risk increases every year. I've recommended many aging players to go for more forgiving frames and lower string tension as they age, me included.
Poor technique can be a cause and one-handed backhands combined with a light-weight, stiff frame, with the wrong strings and tension can do it. Tennis ball can cause elbow injuries as well. As a kid, I tried Tretorn balls, only once, and had my first lesson in getting arm problems from equipment. The tennis ball type has to match the surface and altitude.
The Pros job is to guide students in the proper choices to avoid elbow injuries.