Unfortunately, the entire body must be fully immersed in order for tevilah to be valid. In this case, one brief total immersion is sufficient. She can (loosely) hold onto the railing while going under the water, and she does not have to lift her feet up off the ground. If necessary, she can have another woman accompany her into the water if that will help her. We hope therapy will enable her to overcome this challenge.
Another suggestion (if she is using birth control) is to use the pill to lengthen her cycles. There is no obligation to become niddah and immerse on a monthly basis, and this way she can get her period every two or three months, thus minimizing the frequency of immersion.
Please feel free to get back to us with any further questions.
B’Hatzlacha!