Answer:
In general terms, given that psychology is the study of how the human mind works, how it originates, develops, grows, and interacts with the environment and with other minds, saying that the study of this field is easy, and simple common sense, shows ignorance. There is absolutely nothing easy, or common sense, about the human mind, and even those who say, like in this case, that it is just "common sense", do not know themselves at all.
And in research, this blatant ignorance is being further displayed by the friend, when he says that psychological research, and its findings, are also common sense. From the very first theoretical roots of psychology, to the very last, what is today known is that the human mind, and discovering how it works, is still not fully understood and this is why psychology cannot be taken as "common sense". What a lot of researchers have come to find with their studies on the human mind, is that knowing what is being done, and why it is being done, is paramount to getting proper answers, and this is vital because psychological research guides most other fields of study, as it pertains to explaining how we, our minds, work.
If we just take the topic of biases in research on psychology, then we can show this friend how wrong he is. For example, there are many forms of biases in psychological research that can happen, and which, if the researcher is not aware of them, and how they play a role in final results, wrong theories may be given on how our mind works. One such bias is the response bias, where, a tool used to measure certains factors is done in a way that responses are not unique to the respondent but geared to satisfy a specific requirement from researchers. This will yield wrong results. A second bias, among the many that exist, is the samplin bias, in which, a researcher may choose the wrong size of a sample, and not represent the population, or select people in such a manner that the candidates are not in any way representatives of a larger group. This bias will ensure that results are absolutely invalid, and may cause more harm than good.
Like these two, there are many more biases that can happen to people who perform, or read, psycological research, and do not have the necessary knowledge to apply the proper techniques.
