Physical holdings in gold or silver ("collectibles") are subject to a capital gains tax equal to your marginal tax rate, up to a maximum of 28%. For most people that simply means it's taxed as ordinary income.
Try this tool, for estimating tax due: https://turbotax.intuit.com/tax-tools/calculators/taxcaster/?s=1
Enter your regular income first to see the regular tax. Then add the sale to see the effect.
Enter the estimated capital gain as other income (not as a capital gain). The tool is not sophisticated enough to apply the 28% limit. So, if she is a high income person that will not work. But for higher income people, just assume the gain will be taxed at 28%.
As others have said, the amount of income that she will have to pay tax on is the increase in value (capital gain) from the date of her father's death to the date of the sale. Historical silver prices are readily available on the internet.
