That sentence would have been "Li ne amis sian propran antaŭan edzinon." The word "sia" is reflexive, but does not stress the ownership in the way "his own" does. Using "propran" or "His own" implies there are other wives in consideration that are not related to him. This sentence only implies that he has a previous wife and possible a current one, but it doesn't imply the existence of any other wife.
Jes, tamen se oni devas traduki la Esperantan frazon en la anglan, "former" estas pli bona vorto por "antaŭa".