Yes, it is quite common to have headaches from a stroke. Often, the cause of the headache is unclear, but evidence of bleeding at the site of the stroke and/or
a problem with one of the arteries that supplies the brain in the area of the stroke should be ruled out by imaging (which she most likely has undergone). In your mother-in-laws case, the swelling is the most likely cause. Like any bruise, strokes tend to swell, and reach a peak of swelling about 3-5 days after the initial event. After that, the swelling usually recedes, as should hopefully the headache.
Depending on her age, morphine might be a bit strong. Other options would include 1000mg of Tylenol or 600-800 mg of Ibuprofen. Morphine may overly sedate her or make her nauseous. At worst, it could sedate her enough that she aspirates some saliva and gets an "aspiration pneumonia". I would check with her doctor to make sure that the morphine is entirely necessary.