Gorgo said...
You are obliged to help anybody who needs it as long as you don't put yourself at risk. It's a rule. (I had a quick look at your profile and from the list of sports you have I'm surprised you have to ask the question.)
You're not obliged to be an unpaid instructor, but you should step in if a quick comment from you can save people from grief.
I asked because this forum (despite all it's ups and downs) does have a large knowledge base and you can get all sides of the story. And in many of those sports there are different rules for safety (ie BASE jumping- Help anyway you can, Skydiving- Help only if you can do it safely, Skateboarding-Most of the times you can't help, BMX- help as long as they won't turn around and stab you in the neck...)