I am curious as to the timeline you mentioned. You mentioned your grandmother passed (sorry for your loss) two years ago (maybe I misunderstood), why did she not come back earlier (no need to get into personal details, I am just curious).
As @Crazy Cat pointed out, she should be allowed entry with the potential of being detained, or given a notice to appear before an immigration judge, but I would file the I90 for her GC renewal first so she has that NOA with her to better prove her intentions to repatriate after taking care of her mother’s affairs. The current administration appears to be cracking down more on these scenarios (anecdotal stories), but she does have a pretty good reason for being out so long. Best case, she is allowed entry with instructions as to how to apply for a re-entry permit, worst case, she is detained and sent before an IJ.
Good Luck!