I'm traveling to the USA in December. I'm a student visiting her female friend of 6 years now. The problem is it's the friend paying for the plane ticket and all expenses of the trip. My problem is what to do when asked to provide proof of sufficient funds at my visa interview. Is there some kind of suggestion on what I can do?
You need to show evidence that your friend is willing and able to support you eg an invitation letter, her bank statements. https://travel.state.gov/content/travel/en/us-visas/tourism-visit/visitor.html However, the onus remains on you to meet the requirements of the visa. Not being able to cover the costs of your trip yourself may result in a refusal.
Edit (thanks @Henning Makholm for the prompt): Your documentation must be submitted with the application. US immigration wants to see and assess the documentation in advance of the interview, TSE gets regular questions from people who were surprised that they were refused after a very short interview where they were not invited to supply any of the documentation they had ready.