Is it obligatory to fulfill a promise made to help a "prostitute" repent, especially if not fulfilling it might harm her, given the doubts about the sincerity of her intention?

There is no obligation to fulfill this promise, as fulfilling a promise is recommended (mustahabb) and not obligatory (wajib (definition)) according to the majority of scholars. It is preferable to give charity to the righteous poor among Muslims, as they have a greater right to it. While it is permissible to give voluntary charity to a dissolute person or a disbeliever, obligatory charity should not be given to a disbeliever. One must distance oneself from that woman to avoid falling into her trap.