Yes, Vitamin C can increase oxalate and cause kidney stone, but before this can happen, you must know the person did TOO much.
That means, the person is most likely on >1000mg vit c or its equivalent for a long period of time AND the risk increases with dehydration.
We are now even talking about the vitamin c in oranges. Orange that has a low vitamin C content.
One orange has a lower vitamin c content compared to a single 100mg vitamin c tablet. An average value is 1 orange = 50-70mg of ascorbic acid depending on the source.
What can happen to her is diarrhea (and that should resolve in a day or two), stomach upset, bloating, and urination.
The only need to worry is with people with history of kidney diseases like kidney stone.
Overconsumption of vitamin C (found in oranges) can increase oxalate levels, which may contribute to kidney stone formation.