How can I add an orange flavour without using orange?
It is difficult to be sure what will work without knowing more about the specific allergies in question, but there a few things you might check, if they may not trigger her allergy.

Orange flower water, also called orange blossom water, is one possibility - this will certainly be orange themed.  You might also look at orange blossom honey, which again gets the flavor from orange blossoms.  Since these are made with the flowers, rather than the fruit, it may avoid whatever is specifically triggering the allergy - on the other hand, since it is the product of the same tree (usually the bitter orange tree), it may contain the trigger anyway.

You might also look at other citrus fruit, if the allergy is limited to oranges - this will not be a perfect substitute unless you can find somethign within the family quite close to oranges, it will actually taste like the citrus you used, but it may be quite good.  I'm not sure if tangerines or clementines (mandarin orange, C reticulatata, varieties) will count towards an orange allergy, they are certainly connected, and will again give a very orange-like flavor, but might be different enough they may not trigger the allergy if it is specific to oranges, of the bitter (C aurantium) or sweet (C sinesis) varieties.  Likewise, you may look at pomelo (C maximus or grandis), the citron (C medica), or the papadea subcategory which seem to be the closest to oranges in the citrus family tree, to see if any of them might be suitible.

Going further afield, sweet lemon variants may make a fair subsitute - either limatta or sweet lemon or meyer lemon might give a good final product even if it isn't specifically orangey.

I saw a recipe that used carrot juice, apricots, and pineapple juice to make an orange-juice substitute, I don't know how the flavors, rather than the appearance, actually matches up - but you can look into it if you think it might work.

I hope you find something that works well.  Good luck.