sktowns

I have an F-1 visa interview at the U.S. consulate in London this Wednesday morning. I am UK citizen and have an offer of admission (plus a partial fellowship) to study a Master's in civil engineering at the University of Michigan. My family is unable to support me financially, so a close family friend (who lives in America) has offered to loan me the funds for my year-long programme. I already have my I-20 and proof of financial support in the form of a signed affidavit, but I am concerned the visa interviewer won't believe my story about a 'family friend' (despite the fact these are the actual circumstances). Will the interviewer give me a hard time about this? What kind of questions should I be prepared to answer regarding the financial assistance? Will my situation negatively affect my chances of garnering a visa?

Thanks in advance for any advice you can provide