Frame Challenge: You shouldn't. You should, instead, ask about how your grandmother is coping and try to support her.
Alzheimer's is an extremely challenging disease to cope with, for everyone involved. If you asker her straight out whether or not she should be nicer and more patient with your grandfather I would bet she would say yes. Knowing that that is the case does not in any way make it easier to actually do, and knowing that she is failing to do something she knows she ought to do probably only makes her feel more stressed and drained.
Consider from your grandmother's point of view: her husband, who was an intimate part of her life for a long time, is now different from who he was (at least to some degree), and those changes are going to continue to happen and compound one another. Any important tasks he used to handle are now dumped into her lap. A lot of routines and habits of communication and living are now disrupted. Instead of enjoying their lives together, as she probably imagined would be the case, she has to be responsible for an adult that can't care for himself properly and that is also somewhat unaware of his increasing limitations.
And so when there are clear signs of your grandfather's disease it may be triggering all sorts of negative feelings for her, along with the knowledge that this will consume increasing amounts of her life at the same time that any plans or dreams she may have had are crumbling.
That's hard to deal with, and especially if they are not in an assisted living facility she's not going to be getting much of a break from that. Ever. That's an incredibly difficult situation, and it's not surprising that her frustrations may boil over, nor that some specific behaviors (which are themselves the most noticeable signs of the disease) might become symbols of the whole situation to her. It's unlikely that she suddenly dislikes your grandfather, or that she has coincidentally taken up bullying as a hobby.
So there are a few possible paths. If you think that your grandmother has simply become a worse person (in effect, whatever the actual genesis is) and is abusing your grandfather then it's important to get external help, probably from a legal or social work angle. If you think that she's buckling under some severe and intense stress, then treating her compassionately (instead of accusing her of failing at an obvious task) is a better way to go.
I've recently gone through a very similar situation with my own grandparents, and just indicating that you can appreciate your grandmother's new burdens is likely to help a great deal. Any further assistance you can offer, such as helping with tasks your grandfather used to do, lightening her burden by helping with housework or other mundane chores, will be greatly appreciated and are probably a good avenue towards the better situation you are hoping for.