Whether you say something or not, depends very much in my opinion upon how well you know this lady.
Anyone who is grieving, and most of those who live alone, do tend to talk too much when with others.
If you value this person's friendship, then try to point out to her kindly, but firmly that she is monopolising coversation to such an extent that others cannot discuss their situation, worries or grieve. Tell her this is affecting the group's dynamic, and ask her to give others more space to talk of their affairs.
You risk her being hurt and offended, but if you say nothing she will, sooner or later, be hurt. Either because one of the others blows up and tells her to stop making everything about her loss and her feelings, or because they will stop coming when they know she is going to be present.
Then she will be left wondering what has happened, and you will, when it is too late to do anying about it, be left having to tell her that her lack of consideration has driven them away.
I was widowed myself 2½ years ago, and obviously I still miss my husband, but if I am to have friends and a life worth living, I must interest myself in other peoples' concerns and not make everything about me and my loss.