With 8 nights I would consider seeing a bit more of Japan if you haven't been there before - Kyoto is probably the most popular choice, and at 2.5 hours on the bullet train, easy to get to.

In terms of shopping in Tokyo, there's lots! I think part of where to stay depends on what sort of shopping she wants to do - Ginza, for example, has lots of lovely high end shops and department stores, but if she prefers bargain stores, electronics, or western high-street stores, there are better areas for those things. I stayed close to Tokyo station, and found lots of shopping to occupy me within walking, or a short train / subway hop, and I also stayed at the Intercontinental Tokyo Bay, which would be very handy for going over to Odaiba, which has a couple of western style malls if that is more her thing.

As someone who appreciates good shopping, I would want a hotel near to where I would shop the most - lugging big bags around all day is a pain (although there are stores that will deliver to your hotel, I suspect they have a limit on distance) so being able to pop back and drop things off is handy, I wouldn't want to be staying out in the outskirts having to haul my shopping around all day.