Your finance may be entitled to VA benefits, which are different than military benefits. Whether or not he is medically discharged, he can apply for VA benefits, and can receive them if he can show he was diagnosed with PTSD while on active duty. If he's first diagnosed after service, there are some extra hurdles. If VA agrees that he is "service connected" for PTSD, he'll be assigned a rating from 0% to 100% that reflects his level of disability. Each percentage equates to a different monthly payment (currently a 100% rating would be just under $30,000 a year. A 10-percent rating would be about $1500 per year). Veterans who are service connected and are shown to be able to work can receive vocational rehabilitation where their schooling can be paid for by the VA. Typically, at least for the first few years, the VA will require examinations on an annual basis, and if a condition improves, the veteran's disability rating (and disability compensation) can be reduced.
I do not know of any reason that he would receive 20K a year base pay for the remainder of his life. He would certainly get medical benefits for treatment related to his PTSD. This is all dependent upon how he would be rated by the medical board first, and then subsequently by the VA. He will have to go before a medical board, this is usually done by a records only board first. They will make a decision as to % of disability, then he can accept that or appeal and have a review with an in-person appearance before the medical board. If he is rated less than 30% disabled, then he is given a medical discharge under Honorable Conditions. He would have the option of taking a lump sum from the Army and then applying to the VA for disability or he would be entitled to the % of his base pay that the % of disability was determined to be. He would not be entitled to medical benefits from the military, however, he would be entitled to VA medical treatment which was related to his service connected disability. If he took the lump sum and then applied to the VA, he would not start getting a check from the VA until the VA had recovered the entire amount of the lump sum that he had received from the Army. If he was determined to be 30% or more disabled, then he would be medically retired from the Army and he would have all of the benefits that come with a normal retirement as far as ID, facilities, and medical privileges are concerned. His retirement pay would be the % of his base pay that the % of disability was determined to be. He could still apply to the VA for disability. Any amount of VA disability awarded would depend on certain conditions as to whether it reduced the Army retirement pay in any amount.
You should seek the advice of a lawyer. JAG may appoint one for you, however, I would recommend a private attorney.
My comments are not intended to establish an attorney-client relationship, are not confidential, and are not intended to constitute legal advice. Proper legal advice can only be given by an attorney who agrees to represent you, who reviews the facts of your specific case, who does not have a conflict of interest preventing the representation, and who is licensed to practice in the appropriate jurisdiction where the legal issue may be filed or in the state where the law applies.