I am with LaRue. There have been times when I have instructed my clients to do an exercise differently from the ‘normal’ way for very good reasons. In fact, I told them that it would be possible that they may be approached by somebody who points out the ‘improper’ form.
If I am at a gym and see somebody do something incorrect, I would only approach if there was risk to life or limb. Also, with the realization that some guys look at me with a ‘what do you think you know, lady’-attitude, I have in the past approached the gym owner to point out the situation on the assumption that nobody enjoys calling 911.