Even though on the surface EU and UK are alies and trade partners, it is not all black and white. I believe US will benefit because they will be able to play UK against EU and vice-versa to gain somehow. Say if EU doesn't allow import of US chicken they can go and try to persuade UK to not allow some import from EU in exchange for some other favor and so on.
Also, I wouldn't be surprised if this strengthens US's stock market in short to medium term, as everyone will flee to US stocks due to perceived volatility.