Actually, there is no risk at all (if you know what's behind the covers).
She is stagnating at Google. She had high ranking high influence positions at Google but over the years has been relegated to a position that has little visibility and control, mostly due to her non execution. This is a great opportunity for her to make some serious cash and be the "top dog" in a kennel of average dogs, rather than the opposite at Google.
Also, even if she crashes and burns at Yahoo, she will get many more opportunities simply due to the brand recognition she has due to her tenure at Google.