Is this the topic you have been assigned? I wholly disagree with the statement, on principle. If reality were truly in the eyes of individuals, nothing would in fact be real, it would all be individual perception. I think this idea goes along the lines of the current pop-cultural notion that there is no such thing as absolute truth. What a dangerous idea.
If I were writing this essay, I'd talk about the dangers of believing that reality is in the eye of the beholder. Consider what this would mean for law enforcement, consequences of bad decisions, and general societal peace.
I think that one of the basic points to be made here is that individual perception orr understanding varies from person to person. This is what makes it in the eye of the beholder. The individual sees reality in a different lights, through different points of perception. Psychology plays a large role in this. Different experiences in one's own life helps to contribute to how they see or perceive reality. Those who have understood a psychological experience to loom large in one's life will understand reality different than another person. It is here where our psychology plays a large role in creating the lens through which reality is understood. I think that in pointing this out, one has to also argue that the freedom with which we have in the modern setting allows individuals to see reality different than other individuals. For example, one can see reality in accordance to their own subjectivity. Individual dreams and animating notions can help individuals construct their reality in a different way than others, and in this, one sees how reality can be seen differently in the eye of the beholder.