Immigration officials can deny a visa for any reason or no reason. I’ve known of a few people they have done this to over the years. I agree it makes no sense because of “immigration risks” but illegals can do it and no one bats an eye.
I would suggest your young friend go back to the embassy and show proof of a job, bank account, family, etc. Anything to show she has a reason to return and not stay in the US. If nothing else works then just have her cry during the interview (It has worked for a few people)
My brother-in-law is from Honduras and it took his nephew (who was a doctor) 10 years of visa applications before he was finally awarded one to come to the U.S. to visit............and it took all of the above that you mentioned as well as proof of a credit card to get it. (this was back in the 1990's.....)