All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If that sister is unable to use her left hand in cleansing herself after relieving herself, even with the remaining fingers (of the same hand), then it is permissible for her to use her right hand and Allaah does not burden a soul beyond its scope.
The Fiqh Encyclopedia reads: "The Prophet prohibited us from using the right hand in cleansing after relieving oneself , and the jurists interpreted this prohibition to mean dislikeability. Of course, all this applies to cases in which one is not obliged or in dire need of using his/her right hand, for the known (Islamic legal maxim) that reads "necessities make permissible the prohibited things". Therefore, if someone’s left hand is amputated or paralyzed, or was injured, then it becomes permissible for him/her to use his/her right hand in cleansing himself/herself after urinating or defecating without any dislikeability."
For more benefit, please refer to Fatwa 89878.
Allaah Knows best.