Hi it best to avoid foods with high calcium content. It's quite difficult to avoid foods with calcium completely but it's important to try. It is true that calcium binds with oxalate and the forms an insoluble salt that is not absorbed. But this happen to an extremely small amount of calcium and oxalate that we consume. If this were to happen with all the calcium and oxalate then we would all be deficient in calcium and oxalate. It's nature's way of striking a balance between excess and deficiency. When this balance gets tipped off issues occur. Do not take everything you read on the Internet at its face value. You can research foods with low or no calcium content. Also make sure she drinks enough water to keep her urine from getting concentrated. I hope this was helpful. I wish you and your mother good health. Take care :)