I love The Descendants but this subplot is the closest thing to a flaw the movie has. Brian's behavior here makes no sense. He'd have to be insane to start sleeping with Matt King's wife under the circumstances.
Plus he doesn't seem to recognize Matt King, even though he's famous and the guy selling the land, either when he runs past him on the beach or especially when they meet at the house. (Yet his wife said she recognized him from the papers.) When Matt and his daughter come up to the house you'd think the Speers' would be freaking out -- Matt King is here OMG! Similarly when Matt calls to first set up something about looking at a house Brian's office doesn't do anything special to respond. It's just "yeah come by the open house or whatever."
And then it's odd that Matt himself would have no idea who Brian was considering his role in the deal, and how much time he's spent going over it, and the fact that even Brian's wife assumed Matt would know who her husband was. But at least you can sort of overlook that.
All in all it just doesn't add up that these two guys would have no idea who the other was.
While I expected that Matt knew the big fish who were putting up the money he wouldn't know who Brian was because he was just the realtor working for the corporation buying the property. Your right though Brian should have recognized him. Wonderland Exile