No, it doesn't mean that. The gain would not all be taxed at the same rate. Here's an example to illustrate how it works. Suppose her taxable income, without the capital gain, is $30,000, and her gain on the house is $100,000. The first $10,000 of gain would be taxed at 0%. The $30,000 of other taxable income plus $10,000 of the gain uses up the $40,000 in the 0% bracket. The remaining $90,000 of gain will be taxed at 15%. If the gain on the house is enough to go over the top of the 15% bracket, the remaining gain would be taxed at 20%.
In addition, if her modified AGI is more than $200,000 she would be subject to 3.8% Net Investment Income Tax on the amount over $200,000 (assuming she files as single).
