Having a letter of invitation and a round-trip plane ticket does help to explain the purpose of her trip and how she will pay for it.
However, a young, single lady going to visit a boyfriend or fiancé is usually a death knell for any chances at a visitor visa. It just looks like there is a good chance she will get married in the U.S. and adjust status instead of returning.
I would guess that not mentioning visiting a male friend in U.S. would give her a better chance at a visa. But then again, she will have to disclose the purpose of her trip and who will be financing her on her visa application form, and she will have to answer related questions at her visa interview.
It is also important to keep in mind that the visa only gets you on the plane to the U.S. The final authority on whether she is admitted will lie with the ICE agent at her POE. She should expect some tough questioning by the immigration people on arrival.
It will cost $100 to apply for a visa whether it’s approved or not. If you do purchase a plane ticket, make sure it’s refundable.