She is finally exploring her 'doubts'..... all JWs have them.
Be honest if she asks a question but continue to listen with sensitivity.
She isn't alone, many are in her position and between the abandonment and changes most are privately rattled. Unlike other denominations that hold socialization as vital the JWs are limited to FS for socialization and if she isn't participating she will be forgotten quickly.
Bitterness is a bad way to end life.
Try and expand her horizons beyond daily JW life if you can by special planned trips with your mom, you and grandma. A stroll in a local botanical garden, trip to the museum, a day of remembering her favorite life experiences (a memoir writing session perhaps for family posterity) or introductions to the local senior center, getting involved at the library's bookclub, teaching her to use the internet (hey my grandpa is 85 and is very internet savvy), maybe even enrolling in a college course (my grandpa takes a non-credit course each semester).
I also don't think it would hurt to invite her to church with you or your mom. It is just an invitation not a conversion.
Isolation is deadly and socialization is life.
Sending love, good vibes and prayers to you and your family