Knee-high boots don't always work for a professional look, but in this case they do. First, they're brown, which is a bit less harsh than black. Secondly, her complete outfit is demure and not too formal. The autumn color palette keeps each element - dress, sweater, and boots - pulled together. The length of her skirt is so important here. If you sit down on the couch, it's important that your entire thigh is covered, lest you be mistaken for a starlet on Letterman.
I wish the boots fit her a little bit better, to avoid that bunching around the ankle, but overall this is a very effective outfit. If you remember all the, um, attention Condoleezza Rice got for wearing knee high boots a few years ago, I can understand if you're hesitant to try this, but you really can do it!