Totally fine for her to eat grass.
That prescription renal diet though, leaves a lot to be desired. There are much healthier options with way better ingredients (that won't cause muscle wasting like the low protein ones do), although most vets don't endorse them (probably cause they aren't making money off them). Do you know what her lab results (blood and urinalysis) are?
"To close your eyes will not ease another's pain." ~ Chinese Proverb
“We must not refuse to see with our eyes what they must endure with their bodies.” ~ Gretchen Wyler