Earlier quoted context omitted.
Stallman and the FSF see closed source software as unethical. Imagine if someone asked you for help and said "I don't see how to free my slaves while making a living, paying my rent, paying for my children's education". Would you tell them "You shouldn't have slaves, it's unethical" or would you try to help them find a way to run their business with fewer slaves, but still some?
What am I supposed to say to that? Proprietary software is not slavery, or even anywhere close to it, even if we accept the argument that it is unethical. I find this entire analogy unhelpful.
That means in this case it doesn't make sense to complain and argue about his answer to the developer, as his answer makes perfect sense given his premises. If you want to disagree with his advice, you have to disagree with his moral position on proprietary software and argue about that.
Expecting Stallman to help someone make money with proprietary software is like expecting a vegan advocate to help someone kill cows more humanely.
Note that I'm not expressing an agreement or disagreement with Stallman's worldview, only that his answer is exactly what follows from his worldview, and you should argue about that and not about the answer.