My memory is hazy so sorry if this isn't helpful, but I know a really good restaurant in Shinjuku called Asian Kitchen. It's on the upper floor of a pretty non-descript looking building outside the station.
You exit the station by heading out the South exit and hanging a left, or just hang a left inside and exit farther along (passing the soup kiosk), but either way you end up at the SE corner near the escalators and Tower Records. Go down to the street, you'll see a Green Pea pachinko parlour. Hopefully you can envision where I mean. You hang a left so you're heading North towards Kabukicho. Along that street there's an island in the middle and it's either bicycle parking or a ramp to parking or something, but you veer right. Look for signs for 'Asian Kitchen', it's either the 7f or 9f floor. Once you find the right building, go in the entrance and hang a left, there should be some gold/silver metallic elevators.
The place is large, it's kind of tropical-themed, there tables and huts, they have alot of food and drinks, and every time I've been there it's been quite lively. Lil pricey but that's Tokyo for you.
Immediately around Shinjuku station there's a small bar/coffee shop I know of that's neat, they have food there too. If you go out the West exit you head down the alley with the huge light-up cube (if its still there) between Odakyu and Keio department stores. It's past the chocolate stores, it's on the left side. I think they do Tapas too. It's kind of a classy lookin place with red walls and it's usually dimly lit, small but not bad.
If she lives in Saitama, Ikebukuro might be convenient because the train from Saitama lets off there. Then again she may have been to Ikebukuro many times, kind of a gamble.
Well good luck with your date!