Been said, but you don't owe the guy anything if you were unaware, and it probably just opens you up to a whole shitton of drama you don't need. Its all on her, and that's his problem, and they are problems that you aren't really solving by bringing it to his attention. There's likely an entire complex dynamic going on in the relationship, he might even know of other affairs or suspect stuff but he is just being "stuck" in the marraige somehow.
Really though, how casual was this thing that you can be seeing her for a year and never realize she's married? Does she live with him?