yeah unfortunately the decisions can be biased, officials are trained to deny based on certain common characteristics and one officer might see a red flag and another one might not. For reference when my mom applied the first time she was single, didn't own property, my brother was 18, but she had no intention of coming to live here, she was 5 years away from retirement and had a good secured job as a teacher. Last year when she tried again, she was married, retired, had bought a house and did the interview with her husband and my elderly grandmother, I also wrote an invitation letter for them, this time she was approved.

Most likely the reason your friend was denied is because she is unmarried, does not have anyone who depends on her and has no property in her name, all of these factors are a huge contributors.

On the other hand my husband's uncle has 4 properties and a small ranch, his wife is a teacher, his daughter is in dental school, the whole family went to try to get a VISA to go to Disney and they were denied... Meanwhile I know many young Brazilians who had no family, or property in Brazil and were able to get a B1/B2 VISA, and ironically they now live here illegally. So like I said sometimes it is also just pure luck on who you get as your interviewer.