I have been in that situation myself more than once. As with most of the other people answering, I have a sliding scale I use to respond to the situation. What it boils down to is that the client who is paying for my time always takes precident. If I am assisting them, I may notice someone else’s ‘bad form’ but will usually not approach that person unless a) my client is on a long break or b) my client’s session is over. In one case, however, I saw someone get stuck under their bench press. My client and I were the only people in the general area and I dropped everything to assist the person. I believe this falls into the “life and limb” category!
I agree with Kurt that if I were on my own time I might try to offer some general guidance, especially if I knew the person was a novice. It can be a great opportunity to drum up new business. When I see poor technique in a member that uses the weight room regularly, I’ll often strike up a conversation (if they seem receptive) and ask them about their exercise regiment and what goals they’re training toward. This usually gives me a chance to give them advice that’s appropriate to their personal situation, especially if I compliment their good efforts. In the few cases I’ve intervened I’ve asked for the person’s permission before talking to them or demonstrating anything. If they say no I just hit the road.