DS has a tree nut allergy & we have found that any tree fruit
that is not "tropical" also affects him.
He has reacted to peaches, cherries, plums, etc.
He has no problem with mango (is crazy for it).
We were told he can eat canned fruit, as the heat process
changes it metabolically (or something),
but he won't eat any of those fruits no matter what (his choice - no problem).
I mention this as someone suggested fruit (which is not a bad thing),
but I would ask your niece's parents about EVERYTHING they are aware of.
God bless you for caring!
I have that that same condition. It is technically called "Oral Allergy Syndrome." An allergist explained it to me: I am actually allergic to certain TREES. Since fruits & nuts come from those trees, they have the same or similar DNA, which is what I am allergic too.
I cannot eat these fruits & nuts RAW. I CAN have them if they've been heated in some way, which includes: cooked, baked, poached, roasted, canned, fried. Heating the fruit breaks down the enzyme or protein I am allergic to.
Your son is right to err on the side of caution to not eat them even if they are cooked, as the allergist warned, I may become anaphylactic at any time to the fruit. It could be the next time I eat it or the hundredth time, or not at all.
Your son is lucky he's not allergic to mangoes. I'd give anything to have a nice juicy mango. Yet, even canned, I notice I still have a problem with it.

Maybe it was just that one company. (Trader Joe's had them in mason jars.) Maybe they didn't need to heat them enough to break down the enzyme to jar them. But, I won't be trying other brands.
I also have to make sure any nuts added to a salad have been roasted or toasted, as raw ones will cause a reaction.
I am also allergic to tomatoes & cucumbers as both are fruit. Also allergic to all melons, peaches, pears, apples, plums, kiwi, cherries.
About the only fruit I can have are berries. (Although sometimes I have a reaction to strawberries. Not sure why it's sometimes.) I can have bananas and pineapple. I haven't tried any other tropical fruit since I have a reaction to mangoes & kiwi.

Basically when I go to a farmers market or green grocer, I see all the fruits I CAN'T have.
