For starters, take a look at the 4473, 11(f), and the instructions to that question:
https://www.atf.gov/files/forms/download/atf-f-4473-1.pdf
Also, the following section of the Indiana Code may provide some guidance.
Indiana Code 35-47-2.5
Ed's advice is good. The 4473 disallows purchase if his treatment was involuntary or otherwise ordered. If he sought it himself and was free to leave, I would think he was OK, by the letter of the form.
Note that I did not say "by the letter of the law". I don't know if NICS has the man's name flagged, as the VA did report and by doing so, disqualify many vets for seeking care for exactly those reasons.
State law forbids you to sell and him to buy, both as felonies, if he is ineligible.
Disclaimer: I am not a lawyer. I don't play one on TV, and I did not sleep in a Holiday Inn Express last night. This post is not to be considered as legal advice. If you need legal advice, you need to contact a competent attorney (several on INGO!!) who is licensed to practice in this state.
Blessings,
Bill
18 U.S. Code § 922 - Unlawful acts | LII / Legal Information Institute(d) It shall be unlawful for any person to sell or otherwise dispose of any firearm or ammunition to any person knowing or having reasonable cause to believe that such person—; (4) has been adjudicated as a mental defective or has been committed to any mental institution;