The answer by @Digital fire is not always correct. Some states have passed general-purpose "duty to rescue" statutes. The one I've been trained about is Vermont's
(Cite as: 12 V.S.A. § 519)
§ 519. Emergency medical care
(a) A person who knows that another is exposed to grave physical harm
shall, to the extent that the same can be rendered without danger or
peril to himself or herself or without interference with important
duties owed to others, give reasonable assistance to the exposed
person unless that assistance or care is being provided by others.
The statue goes on to provide good Samaritan protection and state the penalty for violating the statute: a fine of not more than $100.
Digital Fire does refer to the Wikipedia article about a duty to act/rescue, but the lead of that article says it is referring to torts, that is, whether the person who wasn't assisted (or his/her estate) could bring a lawsuit against the person who didn't help. But the Vermont statute creates an offense with which the suspect could be charged in a criminal trial.