The Apple watch is a great gift mate. But for exercise it doesn't mean anything. She will only like the gift if she is techie about her calories burned during cardio. But if she trains with weights, it's useless.
I personally don't use any Fitbits or calorie tracker things like that. If shes into exercise I'd get this book for her as a gift: Starting Strength by Mark Rippetoe. It's a top seller on Amazon and I use it as a personal trainer for training my clients.
I've heard a lot of negative things regarding apple watch. Some of my friends who have been using iPhones for the past 10 years or so and decided to get a smartwatch to keep track of their activities had to buy an android phone along with an Android-based smartwatch. They say it works much better compared to Apple's. I know they wouldn't lie, so from my perspective, I'd not recommend one. Moreover, the android one is cheaper, too!
Its an extremely expensive alternative to a lot of great products IMO. I've got bias as I've never been a fan of apple, but besides interconnectivity with your phone, what can an apple watch do that a fitbit or garmin watch can't?