My advisor and another student collaborated on a paper, after reading it carefully myself, I've come to the conclusion that the approach is fundamentally flawed and the main result is plain wrong.
As recommended in these instances, I've tried on several occasions to politely frame my doubts as questions and asked for further explanation (instead of directly accusing the paper to be wrong), but my advisor was not able to give me adequate explanations. After talking to the student author it seems pretty clear to me they have little idea what they are doing.
I wasn't sure if I wanted to get in this mess, but my advisor wants me to do some work building on this current approach, which they still seem to firmly believe is correct. However, I cannot build on something that makes little sense. What should I do?
Update: I have shown the error in the paper clearly, I have given counterexamples, however I still don't think I'm getting through. While my advisor does agree there are issues in the paper, I get the feeling that he is still brushing these off as minor issues.