I hadn't heard of her or the farm before, but then I haven't actively been invlolved in eventing for many years. I did find the website, and it looks to be a very solid place and she looks to have a good resume.
You also might want to check out Morven Park - they have a resident dressage instructor, and two resident eventing instructors.
Asburn is a *terrific* area, there are lots of great people around you. Two come immediately to mind: The O'Connors in The Plains, and Phyllis Dawson in Purcellville. There's also Jo Struby in Middleburg, whom I can recommend personally, and Jimmy Wofford in Upperville. If those names are a little intimidating, I would check out the list of Level I and II certified instructors on the USEA website: USEA - Directory of Certified Eventing Instructors by Level