How we change what others think, feel, believe and do
A Faulty Proof Refutes His Whole Position
Should your opponent be in the right, but, luckily for your contention, choose a faulty proof, you can easily manage to refute it, and then claim that you have thus refuted his whole position. This is a trick which ought to be one of the first; it is, at bottom, an expedient by which an argumentum ad hominem is put forward as an argumentum ad rem. If no accurate proof occurs to him or to the bystanders, you have won the day. For example, if a man advances the ontological argument by way of proving God's existence, you can get the best of him, for the ontological argument may easily be refuted. This is the way in which bad advocates lose a good case, by trying to justify it by an authority which does not fit it, when no fitting one occurs to them.
You make an interesting suggestion, though this is what Sally has told you. By what authority does Sally speak? I don't think she's an expert here.
That all makes sense except for the first point. And as everything else is based on this, I think you're going to have to start again.
A person is only as good as their worst performance and the slightest error gives their opponents a way in, a crack into which to insert a lever and so bring down their whole argument. Journalists do this when they 'dig the dirt' or pick up on minor points and inflate them into national issues.
This is also a salutary warning for your own arguments. Even if you are in the right and deserve to win the argument, you can lose the argument with an illogical or faulty claim or rationale. Sometimes it is better to say little and under-play your position rather than to exaggerate and claim too much.
Argumentum ad hominem means 'arguing against the person' or a personal attack.
Argumentum ad rem means 'arguing to the point' or 'pertinent'.
'A Faulty Proof Refutes His Whole Position' is the thirty-seventh of Schopenhauer's stratagems.
And the big