Why wouldn't they? In the App Store there are all sorts of browsers that compete with Safari (I know that they still have to use the built in WebView, but still), mail apps (including Gmail), that compete with Mail, camera apps that compete with the Camera app, to-do lost apps that compete with Reminders, and even many existing navigation apps, etc.
There's no reason it wouldn't be allowed in if Google complies with the guidelines...
Maps is expected to be the key monetization engine in mobile, sharing mobile ads with Google is anathema.
Maps 'replicates substantially similar content' in IOS. This one has been used quite a few times.
Apple can be pretty arbitrary about apps as we've seen but by and large from a business perspective its one of their core capabilities they will want to own.
Of course I could be completely wrong here, we'll see as this plays out of course.
There's no reason it wouldn't be allowed in if Google complies with the guidelines...