This is a question that is very difficult to respond to, as you have not given details of the qualitative study to which you are refering to. However, let us remember that qualitative studies focus on thoughts, words, and feelings, in contrast to quantitative studies, that focus on numbers, statistics and facts, and so one way that you can approach assessing the logical flow and coherence of a qualitative study is to make sure that the research methods of this study enable people to express their feelings and thoughts. One great challenge of qualitative studies is that it is actually very hard to ensure that people are honest when you want to find out their thoughts and feelings about something. This is one element that you could assess as you examine the study as a whole.
I have included some links below with more information about qualitative research and analysis, so hopefully these will equip you with more information and knowledge to be able to assess the qualitative study you are refering to.