Hey there Sweetie, having your dog put to sleep is a hard decision as I did this in October. I did not feel guilty as much as heartbroken. The dog was my best friend and helped me through so many battles with my depression that it was one of the hardest things to do but I did it for her. She was in pain.
I am sorry about your son's G-ma. That is hard and the best you can do is support your son and pray for his Grandmother.
As far as the so-called toxic friend..........she is not good for you and you would do well to kick her to the curb and just start to live life without her in it. She is not a good friend and feeling bad about it won't change her. Life without her will be less stressful.
Hugs to you dear lady and find the good stuff in your life today.