The gender of the contributor's father's friend is unknown (perhaps the contributor didn't mention it, or perhaps her father hadn't told her), irrelevant, or inconvenient to mention, so "singular-they" is used. One possibility is that the author may be deliberately holding back the information to surprise us later on in the article. "Singular-they" has been used by native English speakers for hundreds of years, almost as long as "plural-they" (although not as long as "singular-you" in place of "thee / thou".

Based on the context, it implies the gender neutrality aspect.

They, them, and their can be used for a single person. Most modern dictionaries have a usage note under the definition of the word they. An internet search for “singular they” will find numerous explanations.

They, them, or their can be used for a single person when you don't know the gender, when you don't want to disclose the gender, or when the gender is not important to the point you are making. It's also much more convenient than writing "his or her" each time.

Hey! We are going to create a new whatsapp group to improve our English skills. There will be people from all over the world. Some of them are expert tutors of different languages. It will be free. We will explore about the culture of different countries. If you are interested, let me know. Have a nice day :)