This post may be a little NSFW. Skip to the conclusion if you're interested, but not in the details as to how students would torture/kill teachers in Cultural Revolution China.

There are a lot of ways that he could of survived. And its not all that unrealistic. We always think that when some sort of massive event happens that literally every single type of "X" person is systematically murdered. This was true with events like the Holocaust; but even with the Holocaust there were still many survivors. And, since the Cultural Revolution wasn't some rigidly organized government program, it was as equally unorganized as it was effective. Its better to understand the CR as a political struggle, not as an organized event like the Holocaust was for Nazi Germany. If you don't know this, then you'd think that all the Red Guards held the same beliefs and worked together. This is false; many Red Guard units broke down into what were essentially gangs, fighting each other in different districts for what more or less amounted to political clout in an environment of uncertainty.

So onto your question. There are a lot of factors at work here. First, the big one is location. Where did your friend's relative live? The CR was definitely a national movement, but it was mainly focused in the Northeast. Research has shown that Red Guards in major southern and Western cities such as Nanjing and Chongqing never held the amount of influence and power that the Red Guard units of Beijing or Tianjin did. This again is because of the nature of political struggle, what with Beijing being the capital. If your friend's relative was a teacher based in Chongqing, for example, he'd of been pretty safe.

Second, what level of education? The most dangerous spot was to be a university or high school teacher, for obvious reasons. Universities mainly were where the radical sects of Red Guards were concentrated. It is true that some university and high school professors were hanged, or beaten to death by their students for being "counter-revolutionaries." But in many other instances (per research done in fieldwork of anthropologists interviewing Chinese in the 1980-90s), teachers could have been forced into humiliation like being forced to bow to pictures of Mao, and kiss the feet of Red Guard leaders, or being kept under guarded surveillance by Red Guards. It really just depended on who was doing the shot calling. Like I said, there was no "general" or leader of the Red Guard movement. These were 20-something kids who were granted huge amounts of autonomy to commit atrocities in the name of Mao. Some of them abused this privilege, others not so much. A lot of times a good beating would be satisfying for the kids, resulting in a few broken bones but not necessarily death.

At the lower levels of education, it was common for a few high schoolers/university students to take the lead of children in grammar school and organize the kids to punish their teachers. These punishments were far less severe, with an army of children cutting teachers hair, spitting on them, hurling insults, and doing things like forcing the teachers to clean the schools while they supervise. Some report that they forced teachers to drink dirty contaminated water near chemical factories/eat nails, which resulted in some deaths. But overall, being a grammar school teacher was many times more safe than the higher levels.

Finally, was he a regular teacher, or a principle/higher admin? Students were much more quick and willing to kill/beat the school principles and administrators, while forcing humiliations upon lower-ranking teachers. Its quite possible the more charismatic and lovable teachers were warned beforehand, and escaped unharmed. Lower-ranking, regular teachers were often subject to other forms of punishment rather than death, such as females having their hair shaved in odd ways, spat on, etc. There were reported instances of students targeting pregnant teachers, and focusing on beating the womb area as to force miscarriage, but not death to the teacher (though that is possible).

You mention that the friend's relative burned any books (probably Western/Soviet ones) and diaries. That was a common thing to do for many older Chinese who were educated abroad (especially America), who retained books they enjoyed written by foreign authors. Them burning these documents would've helped them, as without evidence they couldn't have been associated with foreign influences, which would've almost certainly got them in trouble.

Conclusion

The Cultural Revolution was a nasty event and resulted in much chaos and death. But there are many factors for teachers during this time. The three most important listed are location, level of education, and rank in the school. Students were most vicious in the Northeast, particularly Beijing and Tianjin, while in other areas of China the Red Guards never had any stronghold of influence. Being a grammar school teacher was much safer than the higher levels of education. And finally, administrators and principals were targeted much more brutally than regular teachers. Considering all this, its not unlikely that your friend's relative is alive today.

Sources:

http://ywang.uchicago.edu/history/docs/2001_03_05.pdf

Revolution, Resistance, and Reform in Village China, Edward Friedman