We need a timeline. I wouldn’t think the Army would do anything if the affair only took place after he retired. If he had an affair on Active Duty, then, yes, they could call him back to Active Duty even for just one day to punish him. Would they? Probably not unless someone was pushing it. The Army probably already has The Inspector General watching this just in case. Most Army Officers don’t realize they are Army Officers until they die. Once you’re sworn in as an Officer, your name is in the Army computer until you die. Broadwell is a different story. It would actually depend on her status at the time she had any sexual contact with Petraeus. If she was on Active Duty, then, yeah, I believe they can go after her, if they want to. If she was in weekend status or Annual Training (2 week) status or just ready reserve or “civilian” status, then it becomes much tougher. And like anything else, it depends on how mad the Army is and how good a lawyer she gets.
That's not entirely correct. A retired officer remains on the Army's rolls and may be recalled to the day he dies.
Similarly, all retired personnel, officer or enlisted who are eligible for retirement pay remain subject to the UCMJ, although this is rarely exercised.
An officer who has completed all his service obligations and tenders an unqualified resignation before retirement eligibility is not subject to recall.
She was imbedded with him in Iraq, and wrote a book, called I think "All In"