Your friend really needs to work with his attorney on this. That said, if the wife really is incompetent, then what probably needs to happen is that the court appoint a guardian and conservator to represent her, have them served with divorce paperwork, and try to negotiate some kind of resolution. Your friend should be prepared to have a spousal maintenance order entered against him since it's highly unlikely that the wife will ever be able to care for herself or work.
This answer is not to be considered a response to a specific legal issue in a specific jurisdiction - it is to be considered only a general response to a hypothetical scenario posed by the questioner. For specific legal advice, please consult with a licensed attorney in your jurisdiction.
And can be difficulty at times for someone to divorce an individual that is deemed incompetent. Depending on the severity, the Court may not divorce someone because they may not even know their own name. I would suggest you contact an attorney in the county you wish to file the divorce.
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