Please, no morality issues. I know the alternatives; arguments to give the person against, etc.
I know that there can be criminal liability to assist or in any way to encourage suicide in any way.
Is a person under a legal responsibility to stop someone from committing suicide in their presence?
No, there's usually no duty to help anyone, and someone who undertakes to help someone has to be good at it because they can even incur liability for helping badly so as to cause more injury.
There are certain people, like bartenders who serve alcohol to their customers, who are under a duty to them, and who can be liable for not stopping them from doing something to hurt themselves, like drinking and driving.
But generaly there's no duty to help anyone or stop them from hurting or killing themselves.
PLEASE READ THIS BEFORE YOU COMMENT, EMAIL ME OR PHONE ME. I'm only licensed in CA. This answer doesn't make me your lawyer, and neither do follow-up comments and/or emails and/or phone calls, and you shouldn't expect me to respond to your further questions if you haven't hired me. We need an actual agreement confirmed in writing before any attorney-client relationship is formed. This answer doesn't constitute legal advice, and shouldn't be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue.