If the situation warranted my intervention at the scene of a crime (no one else there to help, someone flagging me down for help, etc.), then I would stop.
I would let them know that I will do what I can as a Good Samaritan, but will not attempt to act any greater than that. I've yet to be involved in a situation like that, but I think the law now requires medical personnel to stop. Not sure. Perhaps the laws vary from state to state.
If a neighbor ask me for free advice, it would depend on the type of advice she/he ask for in order for me to decide if I would give the advice. If it's a medical question, I strongly urge them to contact their physician without delay. If it's a nursing related issue, I might answer it in a limited way so the neighbor wouldn't take what I tell her/him as gospel to act on without consulting the doctor first.
You didn't really specify what kind of advice, so I answered your question as if you meant medical or nursing advice being asked.