I spoke with my friend who had the severe head injury and she asked me to pass along a tip on something she found helpful. There is a medication in the US called Armodafinil that she found particularly helpful in treating some of the impact of the brain injury. Online there is a description: Armodafinil is currently FDA-approved to treat excessive daytime sleepiness associated with obstructive sleep apnea, narcolepsy, and shift work disorder. It is commonly used off-label to treat attention deficit hyperactivity disorder, chronic fatigue syndrome, and major depressive disorder.

I'm guessing her use is "off-label." And she also said that a test to determine if she's getting enough Vitamin D helped because she's getting supplemental now. Both of these things have helped clear some of the fog the injury caused.

Each case is different, so this is just one avenue to explore.