There are quite a few senbei shops in Azabu Juban (about a 40 minute walk from your hotel).
Two of the most famous are
tanuki senbei - http://www.tanuki10.com
1-9-13, Azabu-Juban,
Minato-ku, Tokyo
And
Mamegen - http://www.mamegen.com
English: http://www.sunnypages.jp/travel_guid...ts/Mamegen/477
I'm pretty sure I've tasted samples at both places.

If she's in the area and looking for a taste of Edo, you can visit what is acknowledged as Japan's first Taiyaki vendors - Nakiwaya
http://japaneseperspective.com/nakiwaya-cafe/

A way to get lots of senbei for personal consumption cheaply is to order broken senbei direct from a factory. The senbei shops might be able to help, otherwise, you could buy online before you arrive in Japan and have them delivered to your hotel. I can personally vouch for kakimochi in Utsonomiya (they're the company I use to ship senbei to London, but it's much cheaper for a Japan delivery)
http://global.rakuten.com/en/store/k...ommend_item_en
3kgs for 3,000yen