When tangential evidence is relevant but not exactly on point, you must show its relation, or connection, to your claim. Either a logical appeal or arguing for a particular interpretation of the evidence might do the trick. On the other hand, it might be better to present an outright refutation.
In both cases, the best way to incorporate the evidence is to combine a summary with textual analysis. That is, provide a fair summary of the outside source and then present an analysis, interpretation, or refutation that makes your point.