I'm sorry to hear about your friend! What happened in terms of the brain injury? An accident? Such hard thing. How is he doing? My reading suggests that hyperbaric oxygen therapy has been shown to be beneficial. supposedly it really aids brain healing. Here's a recent study https://www.psychiatrictimes.com/tbi/treatment-traumatic-brain-injury-hyperbaric-oxygen-therapy. it says "HBOT can dramatically and permanently improve symptoms of chronic TBI months or even many years after the original head injury" Personally, I'd go for it with high hopes it helps!
Brain surgery is even more of a difficult area than other surgeries, and all of them are difficult. Because of the complexity of the brain, this isn't a question for a website, it's a question for the best specialists. Research is very difficult to discern the reliability of often. You never know who paid for the study. If you're not well educated in statistics, the data may not prove what the headline says it does. Much research is preliminary, based on very small numbers of people and very short duration. So while anyone including you can Google for research results and get a lot of info, it's really hard to tell how significant that research is for the average person unless you spend a great deal of time reading all the published studies, and even then, individuals differ a lot in their condition and the best studies might not have considered anyone like your friend. So again, my suggestion is to consult the best experts in the field, the best specialists and see what they say. Combine that with your own research. But it's very hard for anyone on here to really know much about such a complex area of the body. All the best.
I 100% agree with Paxiled - this is a question for expert physicians, specifically those specializing in brain injury. There isn't a lot of evidence that shows this treatment to be effective for most medical problems. Here's an article that would be helpful to read before having a conversation with your doctor: https://www.fda.gov/consumers/consumer-updates/hyperbaric-oxygen-therapy-dont-be-misled