Is it permissible to leave a fiancée after learning that she is enchanted and seeing the effects of sorcery on her during ruqyah, or is it obligatory to marry her to help her?

Dissolving an engagement is permissible out of necessity, but it is preferable not to do so if a cure is possible, because dissolving an engagement might affect the girl's reputation, and because fulfilling a promise is recommended. If the girl is religious and you are able to accept her as she is and be patient with her, then be keen on her and encourage her to supplicate and perform ruqyah (Islamic exorcism), and you will have reward and recompense for that.