Why do you use "them" to talk about just one person?

I've made a choice in how I write explanations in PhraseMix which is a little controversial. Sometimes PhraseMix readers ask me about it, like this question: 

This subject ("You've made plans to visit a friend that you haven't seen for a while. You tell them:") says "You tell them". What is "them" ?

I think "a friend" should be one person.

Please tell me why.

Readers are right to question this choice. Many English teachers will tell you that using "them" to talk about one person is incorrect. I know this grammar rule, but I've chosen to ignore it. Here's an eplanation for why I do that.

The problem with English pronouns

English, like all languages, has a few problem points. One of them is with third-person pronouns.

We have third-person pronouns for talking about males ("he", "him", "his", etc.) and for females ("she", "her", "hers"). When you talk about a group of people, you don't have to specify whether they are men or women; you can use "they", "them", "their", and so on.

But what if you want to talk about just one person that may be male or femaie? English is missing a word to use in a sentence like this:

If you've made plans to visit a friend that you haven't seen for a while, but you have to cancel, what do you tell ___?

The traditional solution

Traditionally, English speakers got around this problem by using male pronouns ("him", "his", "he") to talk about people whose gender was not known. Here's an example from an essay written in 1941:

After 1936 everyone with eyes in his head knew that war was coming. 

"Everyone" can mean both men and women, so "his" refers to both men and women too.

However, some people started to feel that it was unfair for the English language to use male pronouns for all people. It made it seem like men were more important than women.

A more complicated solution

To fix the unfairness of always using "he" and "him", some people started to use and recommend "he or she" and "him or her". This was the way that I was taught to write when I was in school in the 1980s:

After 1936 everyone with eyes in his or her head knew that war was coming. 

The problem with "he or she" is that it's too complicated. See how complicated it is in a sentence with just two third person pronouns:

If a student is unsatisfied with his or her test scores, he or she may arrange to retake the test.

This is OK for very academic-sounding formal written English, but it's too much for any kind of popular writing like for magazines or blog posts. It's definitely too much for spoken English.

The current popular solution

These days, most English speakers use "they", "them", and "their" to talk about someone who may either be a man or woman:

Just because someone has a lot of money doesn't mean that you should look up to them.

Let's try to find a station employee and ask them for directions.

A: One of the neighbors came by today.

B: What did they want?

In spoken English, this is really common.

In written English, it's still a little controversial to use "they" and "them" this way. Grammar traditionalists will still say that it's wrong. But in my opinion, there really aren't any better options. As I mentioned above, "he"/"him" is too sexist and "he or she"/"him or her" is too complicated.

Ideally, I would try to avoid using third person pronouns at all unless I know whether the person is male or female. It's often possible to change a sentence around so that you avoid using third person pronouns, but sometimes the resulting sentence is too complicated. When that's the case, I choose to use "they" and "them" instead of writing something that's grammatically correct but hard to understand.