The only reason I am aware of has to do with certain breathing issues such as dyspnea. Does you Mother have breathing difficulties?
During my last hospital stay (April '10) I complained about all of the morphine they were giving me and was told it oxygenates the heart.
One of the most common after effects of strokes is severe or mild pain in various parts of the body. Pain can usually be minimized very easily with pain medication. Many will just require standard pain relief such as an aspirin while others may need something much stronger such as hospital-grade pain relief medications including morphine.
Also, "aproximately 8 percent of people who have had a stroke will suffer from a neuropathic pain caused by damage to neurons in the brain". This pain can be significantly worse than just usual pain experienced on a day to day basis. "Some of the most common medication and drugs to treat this type of pain include: intravenous lidocaine, opioids and narcotics, anti-depressants and anti-epileptic medications".
The treatment given depends on the effects and location of cell damage caused by a stroke. Morophine also helps with cerebral hemodynamics and cerebral oxygen metabolism that would effect breathing, etc. as previously stated.