Concerned about Grandmothers behavior. Any advice?

I have noticed for the past 3 years or so that my Grandmother has turned extremely bitter and hateful towards certain people for no apparent reason.

She makes my grandad's life hell, he is scared of his siblings dropping in for a cup of tea because she has took a disliking to them. She has become very lazy where she doesn't cook anymore and lives off deserts and microwave meals, she lacks with the cleaning but when I offer to assist she declines.

She has also started showing us up when we go out in public, she makes her opinions known and doesn't care who hears her- e.g. we went to bingo a month or so ago and someone called for a house, and she more or less shouted 'oh you B*sta*d I needed 1 f*cki*g number, bloody rob you when I get outside' but there is absolute seriousness behind it. this is not the nan I grew up with at all, she is unbearable sometimes.

I have spoken with my Mother about it (her daughter) and she told me that her grandmother (nans mom) went exactly the same before she had a stroke- she described to me some of the situations and it was almost identical to my nan. she also said she had the start of dementia before she had a stroke which sadly she passed away 8 weeks after.

I need advice because I do not no how to approach the situation.