I have a coworker with direct knowledge of a Military spouse cheating on her husband. What action, if any, should he take?

Tell him, send a letter, leave a note what ever you have to do. My Ex cheated on me while I was be deployed in 03. No one told me but I eventually figured it out. Now all my "Brothers" who knew and didn't have the courage to tell me I have no use for.
Yes telling him may end badly he may be pissed, they may hate you but why would you want him to live with a woman who doesn't love him and is just getting more of his pay and retirement. Will you tell him when he is raising kids that may not be his? Get word to him don't let the excuse givers below talk you out of the right thing

I would look the AR on that one first. It could very well explode in your face

coc and chaplin, let them take it over, i found out the hard way, when it happened to me, she meant the world to me and fell for a preditor

You didn't say wether your friend was military or not so if the Chaplain idea didn't work an anonymous call to the IG could be available if the phone number was available to your friend. Unless he or she is directly involved letting someone that's supposed to check a serious an accusation as this to look into it.

This post also speaks to cheating in the military, in general. IMO, a military spouse agrees to the same ROE as her/his spouse, so I'm not sure why oaths sworn to an individual seem to fall apart under the duress of distance, loneliness or opportunity. And I REALLY don't understand women who look for (supposedly) committed "fiance/husband/spouses" as targets to see if they can mess up another relationship for s*ts and giggles. As far as I'm concerned, anyone who indicates that there is a companion/finance/etc. somewhere, is off limits. No exceptions.

Present his evidence to the chain of command, and they support the soldier. I served during a time when the support wasn't there. I remember soldiers going thru this and the chain of command making statements like"The Army didn't provide you a wife", and when you tried to fix the problem yourself they (Army)had no understanding.theyre way of dealing with the problem was to remove the soldier from the home which doesn't really address the soldiers mental needs

You need to confront you wife, let her tell you side, then you decide if you want to save your marriage or start looking for another wife. Remember that everyone can make a mistake.

I would tell you to get a shovel but watching them suffer would be better, start looking up numbers for chains of command. Burn em

I would contact the Unit and the command and them Handle it with The SM. I would like
to Say It happened to me while I WAS IN THE BASIC TECHNIAL COURSE BUT I HAD ORDERS TO GO TO GERMANY AND SHE TOLD THE DAY BEFORE MY DEPARTURE TO LEAVE FOR GERMANY. I LATER FOUND OUT IT WAS MY NEXT DOOR NEIGHBOR.

Have the coworker mind their business. The coworker is not the morality police. Maybe the husband is a non loving pos