Then she wouldn't have been in the market for an iPad anyway, so it doesn't really cut into Apple's sales.
It's not just about sales. It's about the concept of OS "lock-in" and why the market share land-grab is so important.
If I owned $10,000 worth of Canon lenses, and Nikon released a better camera than Canon, would I take a bath on all my Canon equipment just to switch? Very unlikely, unless the camera was so significantly better.
If I owned several hundred dollars worth of apps, games, and music from the iTunes and App store, how likely would I want to switch to Android and have to repurchase everything? And vice versa for Android users.
Google doesn't (and shouldn't) care about fully monetizing Android right now. Monetizing a platform is easy once you have the user base locked-in. Just ask Facebook. Google wants to pump Android into every handset/tablet on every carrier at every price-point.
Amazon customized the Kindle Fire's OS with the blessing of Google. It's still Android; it still runs Android apps (thus supporting the Android app ecosystem), those apps still serve ads using AdMob (owned by Google), and most importantly: it keeps another person from joining Apple's iOS ecosystem.
Even if a person can't currently afford an iPad right now, and they buy a Kindle Fire, they are still joining the Android ecosystem. Maybe a year down the line, when they can afford an iPad, they'll think twice and get a Galaxy Tab instead.