To what degree is she able to care for basic hygiene issues on her own (ie. dressing herself) Is she generally able to use the bathroom and clean herself without assistance?
She may love the idea but, if the answer to the above questions is yes, what about using an undergarment like depends and cleaning with moistened feminine wipes (just for the duration of the period)
If the above is not an option, does she have the motor skill and grasping strength to use a peri bottle for cleaning and the larger undergarment inserts like those given by maternity wards after childbirth?
I hope these suggestions are of some use even if only as jumping off point.
best of luck,
PS For all your frustrations, and all those to come, always know you are playing a priceless role in her life. I am married to a wonderful man and my parents are amazing people, but when I look back at my life my grandmother's unending, non judgemental, selfless support is still what I think of as my most important influence *she will turn ninety next month