Is her former husband, through a new contract, more entitled to marry a woman divorced with a definitive divorce, or is it a suitor who proposed to her and she accepted him?

An engagement is a promise to marry that can be revoked for a valid reason. It is disliked to revoke it without justification, and it may even be forbidden if the woman's withdrawal is due to another suitor. The former husband has no right to precedence over others if she becomes engaged to another and the suitor accepts. The decisive factor in acceptance or rejection is the woman's own statement, provided she is not coerced. If your daughter has accepted the suitor, then it is preferable not to break off the engagement unless there is a strong reason to return to the former husband, such as the presence of children.