Need Duck advice ASAP!!!!

Hi,
So we have 4 mallards that we got last Easter. They were just about 3-4 weeks old so we kept them close & warm until they were big enough to release to our neighborhood pond. We realized shortly after the release that we have 2 Drakes & 2 hens. They were thriving, eating well, swimming, surviving any wildlife that might be a threat. We started noticing one of the females feathers looking a little sparse & she was always on her own. Never in the water w/the others. We also started noticing one of the Drakes being very aggressive w/her. This aggressive behavior has gotten so bad that the male will not allow this female in the water at all! He's the reason she's loosing all her feathers. We thought they were mating because my kids ran home saying he was on top of her in the water trying to "drowned" her. We laughed & said no they are just making babies. Well tonight I witnessed what we thought was mating but the Drake was so violent towards her & held her under water for a few minutes. My kids were crying & screaming I finally went in after her (crazy I know!!) Anyway this is our first time having ducks but I've now brought the poor hen home until I figure out what to do.
We've really fallen in love with this particular hen & don't want anything to happen to her! Should we find a new home for that Drake or what? Please help!! Can't keep her in our backyard very long!