Summary
The key components of Friedberg’s prediction did not materialize: Trump was based in Florida rather than New York, he did not buy or own a law firm, and credible estimates of his post‑presidency earnings are far below ~$100 million per month.

1. Where Trump was based after leaving office

• In October 2019, Trump formally changed his primary residence from New York City to Mar‑a‑Lago in Palm Beach, Florida.(en.wikipedia.org)
• After his first term ended on January 20, 2021, he went to live at Mar‑a‑Lago and operated his post‑presidency office from there, not from New York.(en.wikipedia.org)

2. Law firm ownership vs. heavy use of outside counsel

• Trump has relied on numerous independent law firms—Kasowitz LLP, van der Veen, Hartshorn & Levin, Blanche Law, McGuireWoods, Squire Patton Boggs, among others—to defend him and his allies across civil, criminal, and congressional matters.(cnbc.com)
• His leadership PAC, Save America, has paid these firms tens of millions of dollars; by 2022 it had spent at least $16 million on legal services in that year alone, and in total more than $60 million on legal fees since creation.(en.wikipedia.org)
• There is no credible reporting that Trump bought or effectively owns a law firm; instead, he is a major client who hires many separate firms and even carries legal debts to them.(thedailybeast.com)

3. Scale of Trump’s post‑presidency income

• Forbes analyzed Trump’s finances and estimated that during his first three years out of office (2021–2023), he generated about $605 million in operating income total—i.e., roughly $16–17 million per month on average, not $100 million.(forbes.com)
• That income came largely from golf properties, Mar‑a‑Lago, licensing, speaking fees, and NFTs; his social‑media venture (Truth Social) actually lost money on an operating basis in 2022, though his equity stake had paper value.(forbes.com)
• Separately, Trump‑controlled entities like the Save America PAC raised hundreds of millions of dollars (e.g., $255.4 million by January 2021 and about $75 million in the first half of 2021), but those are political committee funds, legally distinct from Trump’s personal income and heavily used for legal and political expenses.(en.wikipedia.org)

4. Later windfalls don’t rescue the original claim

• Since his return to the White House in 2025, Trump and his family have reportedly made over $1 billion in profits from crypto‑related ventures and seen large gains from Trump Media & Technology Group, implying very high effective earnings over that later period.(reuters.com)
• However, those gains are: (a) tied to his second presidency, not his post‑presidency phase after 2021; and (b) irregular windfalls, not a documented, steady ~$100 million per month cash income to him personally.

Conclusion
Because Trump was based in Florida rather than New York, did not buy or own a law firm, and his best‑documented post‑presidency earnings are an order of magnitude below $100 million per month, Friedberg’s prediction is overall wrong.