As you know too well, you are pinned in a difficult spot. At 91, your mother may not be amenable to traditional kinds of treatment such as grief therapy. And
she may lack the energy for activities such as volunteering or exercising that help many people get new perspectives on mourning and loss.
And your mother has also lived to an age at which she's likely witnessed many friends, neighbors, and family members die before her -- and that may add to any feelings of despondency and isolation.
Still, for people of any age and stage, time helps ease the pain of losing a loved one. In the meantime, gently help her recall and replant the good times your mother and father shared -- perhaps by looking at family photos together. Older people who lose a spouse are often particularly plagued by a sense of regret and guilt over missed connections and mistakes, which can make grief more difficult to process.
Then slowly help your mother reconnect to people and activities that will help her become hopeful about going forward in her current life.
Bear in mind that you will likely need to make plenty of time to listen to reminiscing, provide company to ease her new loneliness, and to ease new feelings of insecurity and vulnerability she may have -- especially around issues your father may have traditionally handled, such as finances or housing.