Learn Spanish (Latin America): La editorial engañó al lector, lo cual es inaceptable. - The publisher deceived the reader, which is unacceptable.

If you used just que, it would refer to the noun immediately before it, which would mean the reader is unacceptable! To refer to the entire preceding situation (the fact that the publisher deceived the reader), you need a neuter relative pronoun like lo cual or lo que.

Could we use lo que instead of lo cual in this sentence?

Yes, lo que and lo cual are interchangeable when referring back to a complete idea that was just mentioned in the exact same sentence. Both translate to 'which' in this context. The main difference is that lo cual is slightly more formal and cannot be used to start a sentence, whereas lo que can.

Is editorial a false friend? I thought it meant an opinion piece in a newspaper.

It can mean both, depending on its gender! When it is a feminine noun, like la editorial in this sentence, it means the publishing house or the publisher. When it is a masculine noun, el editorial, it means an opinion article.

Why is it al lector and not just el lector?