I believe the standard set on 4473 is "involuntary commitment" by a lawful authority which I've always thought to be a very fair standard. I wouldn't want to keep people from getting the help they need under threat of losing their rights (which could happen with any mental health history [e.g. PTSD, feeling "depressed" not to be confused with depression]). And if someone is dangerous enough to warrant confinement then they probably aren't safe with guns.
The wording of 18 USC 922g is "who has been adjudicated as a mental defective or who has been committed to a mental institution". Whether or not he lied on the Form 4473 he certainly should have had membership in the NICS naughty boy list.
Te occidere possunt sed te edere non possunt nefas est