I rather suspect that the TBI issues have already been getting worse. The letter writer doesn't say how long they've been married, but it's been almost forty years since the accident. Did the letter writer know about the issues before they married? If so, I'd argue that helping with them was part of the commitment. How long have they been married? If the marriage is recent, how did the wife deal with this stuff before? If the marriage is older, well, the same question still applies.

Also, how long has this been an issue? If it's relatively recent or has been getting noticeably worse/different, then seeing a neurologist is absolutely and immediately necessary. (Though, yes, possibly not economically feasible. Although, it sounds like the wife might qualify for disability which would mean some level of health care coverage.)