Seems like you can't do anything medically for your friend, but if he brought his condition to light, it would help the statistics and research, which could then help politically in preventing it from happening to other people.
https://clinicaltrials.gov/ct2/search/index
The care you receive during a clinical trial is, of course, top notch.
Speaking more anecdotally, I have a few close friends that also received a lot of brain injuries in their youth due to sports. Their progress, as you say, is spotty and individual. Personally, the ones that have made an effort to improve themselves mentally have been the most successful to any degree. When it comes to memory, saying that you just have a bad one will likely let it remain as such. Working on memory will help, if slowly or at all. Things like notepads, journaling, meeting notes, and a 'no excuses' attitude were key methods to friends that made progress.
But, as you say, the brain is awfully murky and nature kinda has to take her course. Nothing is likely to stitch back together the neurons that were lost. However, the brain is awfully murky, and there seems to be nothing stopping it from rewiring the neurons that are left.
Best of luck!
Unfortunately it’s an ugly idea. But it may work.