Marcia: Not all vegetarian diets lead to nutritional deficiencies.
Marcia's Conclusion
Some vegetarian diets don't lead to nutritional deficiencies.
Marcia's Evidence
Research shows that vegetarians can get all the proteins/minerals they need from nonanimal foods.
Theodora's Conclusion
You're wrong to say vegetarianism can't lead to nutritional deficiencies (i.e. vegetarianism can lead to nutritional deficiencies)
Theodora's Evidence
If most people went veggie, some people in the meat industry would lose their jobs, fall into poverty, and hence not be able to afford a nutritionally adequate diet.
Evaluation
Where to begin, Theodora? First and foremost, out of the gate you accused Marcia of something she never said. Marcia said that some veggie diets aren't nutritionally deficient. She never said that if you're on a vegetarian diet, it's impossible have a nutritional deficiency.
This type of inappropriate response is a somewhat recurring flaw we might call Irrelevant Response or Straw Man, in which an author is responding in a way that feels totally inappropriate to the original position.
We might also critique Theodora's use of indirect causation to prove direct causation. She is trying to argue that vegetarian diets can lead to nutritional deficiencies. We would commonly understand that conversation to be about whether the ingredients of a vegetarian diet lead to nutritional deficiencies for the person on that diet, whereas Theodora is stretching that conversation to include the eventual economic consequences that could affect people who might not be vegetarian.
This is an Equivocation based on the idea of "lead to". In Marcia's sense of it, she meant would the consequences of a world in which more people ate vegetarian result in anyone having a nutritional deficiency"?
Goal
Either look for an answer describing Theodora's false charge against what Marcia supposedly claimed, or an answer calling out Theodora's abuse of stretching the concept of "this diet leads to nutritional deficiencies".