Shabos 13a discusses the teaching that a Parush Zav and an Am Ha'aretz Zav may not eat together. The Gemara concludes that the Parush in that teaching is not a Zav. Hence, the reason the Parush may not eat with the Am Ha'aretz Zav is he might inadvertantly eat Tame food from the Am Ha'aretz Zav.
However, just before this we learned the reason why the Mishna states that a Zav and Zava may not eat together (and not the case if only one is Tame). We learned that if only one is Tame, they would not eat together in any case because they were careful regarding the laws of Tuma. Therefore, the Mishna had no need to teach us this. If this is so, why would we need to learn that a Parush and an Am Ha'aretz Zav may not eat together? Wouldn't it also be true that they would not eat together in any case?
Rava's answer, which you are referring to, is, "Perhaps he will become accustomed [to eating] with him, and he (the Zav Am ha'Aretz) will feed him (the Zav Parush) Tamei foods when he (the Parush) is Tahor." The intention is not that the Parush, when he is Tahor, will eat with the Am ha'Aretz when he is still a Zav, but that he will eat with the Am ha'Aretz when the Am ha'Aretz is Tahor. Nevertheless, even though the Am ha'Aretz is Tahor, all of his foods are Tamei by virtue of the fact that he is an Am ha'Aretz, and the Rabanan decreed that all of the foods of an Am ha'Aretz are Tamei even when he is not a Zav.