You wait for him to break up with her, but don't wait around for him because you don't know how long that will take. You don't want to be the other woman as you may remain the other woman if he isn't serious about you. Some guys will say anything to get with you a few times while remaining with their girlfriends/wives.
Do not be so quick to Buy into it, Biting on this here, dear, Nor... Wear your heart on your sleeve. He could be having some Troubles in Paradise with her, be even in a Relationship Rut, where Another is a bit more Amazing and New with Someone like You. Be friends only for now, Stay on the Sidelines and be Light and Semi sweet. You do not want to End up in this "Triangle Threesome," where you are Way at the top, looking down, Side to Side, at the Both of them. Good luck. xx