If you saw one of your Joes doing this, how would you react?

I went to basic with this SM and saw on his Facebook that he went to Prom in his ACU's. How would you go about mentoring one of your subordinates that are in violation of AR670-1?

AR670-1 Chapter 4 Section 3 Line 2... That's what I found in the reg, please attach anything else you find

**EDIT** This is not a Dance, but it is some Formal assembly and to the best of my knowledge he is not on Duty.. Class A's would of been appropriate (as long as it's in regs, which I'm sure isn't hard since he would only have 1 ribbon)