What is the ruling on a woman who is unable to wash her feet during ablution because she cannot dry them, so should she abandon ablution and perform tayammum, or should she omit washing her feet, or what?

Your grandmother should wash her face and her hands up to the elbows, wipe her head, then perform tayammum instead of washing her feet if washing them without drying them harms her, and she cannot find anyone to do so. However, if she finds someone to dry them for her, then she must perform a complete ablution.