That's not a serious issue and, going by US law at least, you're never obligated to intervene to save someone unless you created the situation that threatened them, have a specific special relationship with that person (i.e. parent), or have accepted a role where duty to rescue is part of your obligations (i.e. firefighter).

As an uninvolved third party witnessing some harm you will face no penalty for not intervening.

It is, of course, encouraged to intervene and laws have been written to ensure that a reasonable effort to intervene won't cause you legal issues but you are never obligated to do so.