sugar free natural food that take a long time to digest... oats are apparently very good... salads with beans and other pulses,,, wholegrain cereals generally and fish or meat anything that releases its energy slowly to avoid sudden highs.
there are lots of different foods that are good and lots that are bad.
Pls ask her GP to refer you to the diabetic dietician asap usually at your local polyclinic and they will give you guidelines to follow and plenty of leaflets etc if the sugars are that high she should also be seeing a diabetic consultant at your local hospital she will need help to get her on the right path.
I admit to not knowing the whole story, but it would seem to me that the answer is not so much as to what foods lower the blood sugar, but concentrating on not eating foods, including alcohol, which will exceed her calorie quota.
Also, not to miss her insulin shots and maintain her exercise levels.
Also, have a look at diabetes uk website, they have lots of dietary information and are the 'go to' site for lots of very knowledgeable and useful information about all sorts of things.