Does wearing just one piece of a uniform equal being out of uniform?
14–4. Occasions for wear
The PFU and the IPFU are authorized for wear on and off duty, on and off the installation, when authorized by the
commander. Soldiers may wear all or part of the PFU or IPFU with civilian attire off the installation, when authorized by the commander.
AR 670-1, 3 Feb 2005.
Well that's what the reg says.
The real question is: why even chance it? Did your Soldier have no other shirt? Do you know your post's policies regarding uniform wear in on-post gyms?
My recommendation for a situation so trivial is to back up the other NCO. Since you posted the question, clearly you're not 100%.
Ii applaud your efforts to support your Soldier. However, in this instance, your actions could be interpreted as undermining an NCO senior to the situation. Next time, walk up to the situation and say, "roger, we will correct it."
Trying to better interpret a regulation or command policy with another NCO is an action you handle one-on-one; certainly not in front of subordinates.
You have to look at different things though. It might not be in an Army Regulation but I am sure it is stated somewhere. The last couple of units I have been in have what is called the Blue Book. It is a book of additional standard that Soldiers in that Unit must follow. Remember a Leader can always add to but not take away. Here at JBLM we have the 7th ID blue book. It states in there you can not mix civilian and military attire. It also states that you cant take off your blouse while driving in a POV or wear a ball cap.