Maybe yoga or something to increase body awareness? Or just get her to relax her leg when riding. Sitting trot may help he eventually relax into the tack as I've found you only bounce worse when you grip.
She is a triathalete and very muscular and feared her muscles mass was making her grip - I guess that wasn't well stated. Yes, sitting trot and bareback are good ideas for her. I have a pony she can ride to help her out. Also trying to find a new instructor!
What a thoughtful question - although learning to be more sensitive is quite the challenge!
A friend had an aha moment that her leg aids were too strong for a sensitive horse. She is learning how to relax and feel with her leg aids - instead of powering through.
One of the best pieces of advice for me has been to practice a new riding technique through out one's day - driving, sitting at work, etc. Focusing a few minutes through out the day learning to pay attention to leg aids will help create a new habit.
Two point is meant to get a STABLE leg (not a strong one per se), and one which where the rider is 'feathering'/stretching into the heel. By doing this all the joints (hip/knee/heel) are elastic. And just like a rubber band pressed down around a curved object it 'adhedes' to the surface. This is the opposite of strength in gripping the horse. The best rider I have is a marathoner/ironwoman.
Aside from two-point this is somewhat the same concept. While positing to the trot, she could try rising for two steps, and sitting for two steps. This may be hard on the horse after a while, so don't do it for long periods of time but it's helped me with my leg majorly.
If her leg isn't still and if she's using her thighs instead of her calf muscles it won't work out as well.
Just something I've heard about..
Its possible, especially if like me she has a lot of muscle mass in her thigh. Having a lot of "meat" in that area definitely makes it more difficult to get down and around the horse.
I think if she feels like she's gripping too much, lots of two point practice will help get the feel of having that drapey leg with heels down. As for feeling that the aids she's giving are too strong? I think that just takes some time and playing around with the horse to get to know them. Is she only doing lessons, or is this her horse?
And one last thought...she might feel the need to grip with her strength because she doesn't have a great seat/balance yet. I remember doing that a lot when I first started before I figured out that a lot of staying on top of the horse was not necessarily based on how strong I was. So perhaps some lunge lessons to help her get more secure in the tack without feeling like she has to have a vise like grip with her leg?