As a substitute teacher in a highschool how would you respond to a student getting dress coded when there's no dress code?

I've looked all over the district handbooks website etc. I cannot find a dress code for students. However while headed to clock out & leave I witnessed an older staff member fussing at a girl about her shirt. There was absolutely nothing wrong with the shirt it was plain black v-neck. Not super low. The girl just happened to be blessed with boobs. The staff member and older lady was saying "I better not catch you wearing that shirt again". There were tons of kids around & it was hectic & I didn't know how to respond & I'm not at the highschool a ton & have no idea who that staff member was. What would you do? Dress code bs really upsets me & I wish I had stood up for the student but had no idea what to do in the moment & honestly no idea what I'd do in the future.