If your fiancé was convicted or found not guilty by reason of insanity then this felony does fall under the 28 automatic disqualifiers. Honestly, I don't think an airline will even consider him. But that doesn't mean he can't fly at all. These disqualifiers pertain to those who are granted unescorted access authority, meaning they are issued a SIDA (Security Identification Area) badge. These badges are issued by the airport police who are operating under TSA regulations. This is a TSA requirement and is set forth to protect the flying public. It may be possible to find a job that does not require a SIDA badge - perhaps some of the folks on the forum can give some suggestions.
If he were my fiancé I would try to encourage him to stay flying as a hobby and perhaps pursue something else within the aviation industry that doesn't require an FBI background check. That may save him a lot of frustration in the long run.
Best of luck,