In the U.S. (as well as most Common Law jurisdictions the world over) there is no "Duty To Rescue" that is universal to all cases. Such a duty only arises under certain conditions specific to the nature of the incident and include:
When the person creates a hazardous situation (I.E. In a car accident, regardless of fault, both drivers have a duty to rescue.).
Parents, Legal Gaurdians, and in loco parentis of minor children.
Common Carriers to patrons (Such as Captain Sully walking the plane back and forth looking for any passengers who had not evacuated before he left during the Miracle on the Hudson)
Employers towards Employees
Real Property Owners to Invited Guests and Social visits. In some jurisdictions this is extended to trespassers, though in the U.S. this is not a hard rule.
In 10 states, one has a duty to notify law enforcement when encountering a stranger in need of rescue assistance (not including the aforementioned Minnesota, the other nine are: CA, FL, HI, Mass, OH, RI, VT, WA, WI).
In all cases, duty to rescue does not apply if in the course of the rescue the rescuer is exposing themselves to danger (So if Hitler was in pool and being electrocuted by a downed electric wire the Life Guard could refuse to rescue because jumping into water that is conducting an electrical current is a risk to the Guard, even though he would have a Duty to Rescue as he is a Common Carrier and Hitler is his patron.). It would be wrong if Hitler was refused rescue from a different situation because the Life Guard was Jewish.
In your initial scenario, Alice does not have a Duty to Rescue Hitler from the outset, however, there is a scenario where Alice's Duty to Rescue could change. Depending on what methods she uses to convince Bob that Hitler should not be saved, she might be introducing a hazard into the situation if she impedes Bob from an initial reaction to rescue Hitler (I just want to point out that this is one of the most unusual out of context sentences I've written on this site.). It would matter how committed Bob was to saving the life of a man he may not have known was Hitler OR if Alice took physical actions to impede Bob from his decision to rescue Hitler.
Bob would be liable if and only if, once he began to actively save Hitler's life, Alice makes an argument that convinces Bob to cease these activities under his own volition. This liability could be arguably shared with Alice, as both created a hazardous situation for Hitler (Bob by his cessation of an ongoing attempt to rescue and Alice by her argument. But For Bob's refusal to render further aide, Hitler would have lived. But for Alice's arguments to Bob to let Hitler die, Bob would have continued to save Hitler's life.). This one is a bit harder to argue, however, it is something I could see a judge considering.
With out more details into the specifics of your scenario, I cannot give you more than a speculative guess at the answer as it is presented. As such, absent this evidence, I must assume that innocence of wrong doing on the parts of both Alice and Bob unless and until these details are fleshed out.
I will however close with the caveat that in the U.S., what is legal is not always what is moral. In Hitler's Germany, what was moral was not always what was legal. However, when in the course of events, there is a rare intersection between that which is Moral and that which is Legal you will find that which we are and that which speaks loudest of all. Mercy for the devil is mercy none the less. And as one great fictional legal master noted, Mercy is twice blessed.
