She doesn't need agency or third party website to apply for visitor visa.
She just needs to file DS-160 form, upload photo and pay the fees:
https://travel.state.gov/content/travel/en/us-visas/tourism-visit/visitor.html
Make sure she puts true information on this form. Any material misrepresentation / lie / inaccuracy can cause her never get any US immigration benefit.
Many visitor visas are denied, especially if she's a single lady with weak ties to her country.
Things like good job or successful business, properties and other ties increase her chance of getting visa.
Worst case, if she gets a denial, you can meet in Europe as she has Schengen visa and you supposedly have US passport.