If it was just the original question, I would say no, no problems, a 9years old can be trained with great success and no huge issues.
That is, if it's a good, experienced trainer
who does the job. And there's no way to foresee if this particular mare will be a beginner horse or not. Not a clue on the planet. She might be calm and lazy, or she might be hot and need a firm, experienced hand. It doesn't really matter how she does look in the pasture.
But since you added:
She's never even ridden a horse, plus, I already know, she is not going to want to listen to anything a trainer has to say, she'll want everything her own way.
Your friend doesn't really want this horse, unless she's just going to feed her from outside
the pasture fence.
What your friend needs is a 20+, bomb-proof retired school horse, one with great experience with beginners, the kind that no matter what you do, won't spook, won't become aggressive, possibly won't ever go faster than a pleasure-speed canter.
...no horse at all would be even better, 'cause if she's really clueless and wants to do all by herself, there are high chances that she'll get badly hurt, and whatever horse will be sold quickly to whoever comes and picks him up first.