There is no exemption from mikveh immersion for a physically disabled woman.
Some mikvaot have been designed to be accessible to women with disabilities, incorporating devices such as hydraulic lifts. If such a feature would be of use, please write again with the woman's location. We can track down the one closest to her.
Alternatively, a woman can arrange to have her mikveh attendant (or a friend) hold her during immersion. The attendant should dip her hands in the mikveh prior to her immersion and should hold the disabled woman loosely enough for water to penetrate her grasp. It would also be possible for two women to hold her under these conditions. (See Shulchan Aruch Yoreh De'ah 198:28 and the commentators there.)
Another possibility would be for the disabled woman to hold on to an underwater handle or railing, again in such a way that water can penetrate her grasp, or even to sit on one of the mikveh's lower steps. (See Shiurei Shevet Halevi p. 347.)
If none of the above suggestions resolve the situation, we suggest writing back to us or contacting a local rabbi with more details.