Typically, if a WGA prompt triggers, there's an option to click on to contact Microsoft.
However, I'm thinking the odds don't look good in your (or your friend's) favor. I'm thinking the custom build PC had a pirated copy of Windows installed on it in the first place. Otherwise, she would have been given the installation media and/or the certificate of authenticity with the product key on it. However, it's not too farfetched if she had installation media in the past and lost it...
Now if the media you are using has only been installed on her PC (or the previous PC that it was installed on is no longer in commission), then you can go through the "Activate Windows" wizard on her computer. Once it fails, it will give an option to activate by phone. Click that option and call that number. The automated system will ask for a long (and I mean LOOOOONG) series of characters displayed on the screen. If it works, then it'll spit out a series of characters for you (or her) to type in. Once it's done, her computer is activated. If it fails, then it'll transfer you to a representative and you'll have to explain what happened. Typically if the key is only used on one PC, then the rep will give you a series of characters to type in to get it activated. Sometimes they may ask for proof of purchase.
Worst case scenario, the WGA wizard can be used to purchase a new license at a discount.