It is a difficult situation for you ,but at the same time I feel that friendship means taking the rough with the smooth at times. If you think back to how she used to behave, do you think that this new behaviour is to do with her now living a life alone and perhaps letting habits of listening to the news too often means that she is getting upset about things and no chance to talk it over with anyone. Another possibility is if you look at her pattern of behaviour do you thing that she might possibly be ill? A big change in behaviour could possibly be to do with dementia or a tumour for instance. I hope that isnt the case, but if you could talk to your other friend and between you you might consider these changes.
Then the difficulty thing is to find a time when you can both be very calm and hopefully she is in a receptive mood and suggest that she might need to check with her doctor, as you have both noticed these changes. I think that at the least , this is a kindness and care you can offer her. It may be that she hasnt realized how much she has changed if it is happening slowly but you see it more only meeting occasionally.
You may be glad if this means that there is some medical problem and you have alerted her to this. Even if she gets annoyed and flounces off , you have done your best as a friend to help her. If however she denies it all and continues in the same way then sadly it may be time to stop meeting or seeing her, but I do think that old friendships are both something to celebrate and to cherish and deserve what efforts we are able to make. However pleasant more recent friends are , there is no one who knows your history and your past family ties etc. I cherish my own precious friendships especially one with someone I have now known for 78 years as we met at the age of two. We have been there through thick and thin for all our lives and are grateful for each others support and more importantly I feel, is that she remembers my granny, I remember her dad. They were the most important people in our lives and there is no one else alive who remembers them in that way which for some reason I find comforting and cheering. Can you look back at times spent with her where you did have good times together and even if you have to split up with her , at least have those good memories to hold on to.