------------
How do you determine "if she weren't worth it"? Assuming this person has been at the company for a while, she's now only worth something like this relative to the value of Facebook having her on staff. The value the person is bringing to Google was never close to $6m, otherwise she would have already been getting that (or close to it). The $6m value to Google is in keeping whatever she knows away from Facebook.
Also, FWIW, these huge payouts to a few Googlers are primarily stock payouts, not cash. Is this $6m deal just a cash payment? $6m/year salary? This person probably has some very strategic knowledge which Google simply doesn't want Facebook to have.