Leo wouldn't recommend getting a Windows based tablet now because the existing ones are running an operating system not designed for touch. That will change though when Microsoft releases Windows RT, which is pretty much Windows 8 for tablets. Microsoft is expected to put out their Windows RT based Surface tablet in September or October. If she can wait, and if she's really happy with her Windows Phone, she might want to consider these Windows RT tablets. There won't be much software available for it because it's so new, though.
The iPad, on the other hand, has a lot of applications, has been around for awhile, and is beloved by millions. If she were to give him no other information and was just asking what tablet to buy, Leo would say the iPad. There are some strong competitors right now though too. The Google Nexus 7 is an Android tablet and is available for $200. It's 7 inches though, so it's smaller, but that's an advantage to some people. Android also has a lot of apps, and Leo believes there are a similar amount of apps on each. But the iPad is a blockbuster. It was the first good tablet and still considered to be the only good tablet by many. The only reason to really get the Nexus 7 is that it's cheaper.