Assuming she is a US resident, the duty-free exemption for this trip is likely $800. This exemption can be shared by members of a household, so if she is traveling with someone else, they can have a $1,600 exemption between them.
On her flight to the US, she'll complete a customs declaration form listing the value of the laptop. Duty rates are quite complicated, but there's a flat 3% rate for the next $1,000 you have above your exemptions. So if she's traveling alone, she has an exemption of $800, leaving 1500-800=$700 subject to duty, equaling $700 * .03 = $21 owed at the border.
In some cases, they may simply not bother to collect small amounts of duty for travelers at the airport and will wave you through. Otherwise, the officers will direct you to a cashier where you can pay on your way out of the customs area.
Note that if her home state has a sales tax, she may owe "Use Tax" (the same as sales tax essentially, for out-of-state purchases brought home) to her state government. This tax would apply regardless, but many people don't pay it, because the state doesn't normally know what you've bought. However, if you pay customs duty, the state may later mail you a letter reminding you that you owe use tax. The rate will be the same as the normal sales tax rate for her place of residence, just as if she bought the laptop at a local store.
If she's eligible for a refund of German VAT on the purchase (assuming the store offers this service), she should be sure to complete the proper procedure for that before leaving the EU. The amount she gets back ought to be more than the US taxes.