You did the right thing. While it would not be "wrong" to mind your own business, you saved someone from possibly formulating a bad opinion of the shop, or even worse, injuring themselves.
I usually keep my mouth shut when I hear opinions flying around. However, if there is a safety issue or uneducated/faulty info given, I will usually pipe up. (That is if I'm familiar with the subject.) Usually in a sales situation the staff appreciates the info.