I know that there are a lot of well meaning people out there, but getting unsolicited advise is one of the most annoying things in golf. When I am on the range, I like to work on things, and because I may be working on something, my shots can be a little erratic. Infallibly, the guy next to me will suggest lessons, or some piece of insight that they may think that I am unaware of. Sometimes this may be productive, but more often then not it is frustrating and seems condescending.
When I saw this happen to another person a few months ago, his outward reaction displayed everything that I had felt but did not say. My advise would be, if you notice a fundamental flaw in a person's swing, do not rush in to save the day. Instead, start a light conversation with the person, and get an idea of what their experience level is. If the person needs help, they will most likely ask, or otherwise indicate at some point.