This situation has a lot of layers to it and should be analyzed before any decisions are made. If a student is consistently performing poorly on tests (notice this is multiple so there may be a pattern) I take a look at the overall performance on each test by the entire class. If the class performance closely mirrors the one student’s, then yes, there may be a problem with the test. However, if the rest of the class is outperforming the one student on the test(s) then, most likely, the problem is not with the test.
Hopefully, you have this information ready when you talk to the parent. I would be well prepared to communicate all details about that particular student’s efforts in class and how he or she compares to their peers. I would also have an action plan ready to suggest to the parent if they do not offer one first.
In the end, professionalism will be the most important factor in the teacher’s conversation with the parent. Being prepared, polite and proficient should allow the situation to be resolved easily.