Why not play it safe and go eat at John Lewis?
There's an assortment of options there and lots of safe choices - many of which she can see for herself first.

Non upscale Japanese restaurants will have a selection of 'bland' choices - salt grilled fish or meats (or even tofu, perhaps the blandest of all foods) cooked in a variety of ways served with white rice. If you call in advance I'm sure the chef could cook a chicken breast or pork cutlet simply if she doesn't want it breaded and fried or coated in sauce. And there are plenty of vegetables that can be served without strong flavours.
"ten ten tei" -57 Brewer Street would fit the bill. It's unpretentious, not expensive and has a wide selection of dishes.