Is it possible to fight a chapter 14-12b

This is one of those “an ounce of prevention is worth a pound of cure” situations. The best way to fight patterns of misconduct is not to have patterns of misconduct. It’s like when people ask how to avoid getting kicked out the Army for failing APFTs, we’ll just don’t fail the APFT.

You avoid getting kicked out by doing things that don’t get you kicked out. As my old 1SG used to say, we don’t ask you to do anything that you wouldn’t be expected to do in the civilian world: don’t commit crimes and show up to work

Has he tried not being a habitual turd?

I think it's best to first understand a few things about a Chapter 14-12b. I have dealt with this quite a bit. First, it is not a form of punishment. A 14-12b is an administrative separation. It is basically the Army's way of saying that you are not in the Army's best interest to keep serving. I know it may sound crazy but the Army can fire a Soldier. Just because it is the military it doesn't mean that you have to stay. Next, several leaders will review the company commanders recommendation for the separation and review the grounds of the recommendation. They will decide what is the best interest for the Army. Every packet will be reviewed by legal counsel but they aren't there to stop it or to even fight it. They are there only to ensure that you understand what is going on and to assist you in filing a rebuttal. Very few are retained. You really can't fight because it isn't something that you can fight. I have seen Soldiers open door their commanders all the way up to their BDE commander. It didn't work. Their willingness to be a better Soldier didn't negate their actions that led them to the recommendation.

Your buddy made a habit of being a dick and showed misconduct and now wants to fight it after getting called out on his misconduct and being a dick? Stop worrying about your buddy. Your buddy will be provided all of the documentation he needs and a chance to visit a lawyer.

two paths forward. try to disprove or refute the existence of the pattern of misbehavior or find a procedural error, for example if he wasn't given adequate counseling or adequate opportunity for rehabilitation

He should read chapter 2 of the same reg (635-200):
a. You have a right to consult with an attorney within a reasonable time (not less than three duty days) or civilian counsel at your own expenses (your command should make a TDS appointment for you).
b. You have the right to submit statements in your own behalf (these statements may be made by you or by others on your behalf).
c. You have the right to obtain copies of documents which your commander will forward in support of the separation recommendation.
d. You can waive the above rights in writing. Failure to respond (that means not signing the form) within seven duty days from the date you see an attorney means you give up all of your rights; and
e. You have the right to an administrative discharge board if you are considered for discharge with an Under Other Than Honorable Conditions (UOTH) characterization of service or if you have six (6) or more years of service of total active and reserve service on the date of initiation of recommendation for separation.

Your friend has less than six years, and if the command is pursuing a general under
honorable conditions that also means there will be no administrative board for your friend to appear at in person. Commands will typically do this to save time and allow a Soldier to retain VA benefits.

Bottom line, the way you "fight" a 14-12b is by becoming a stellar Soldier when the counseling and rehabilitation starts, not when the chapter starts.