He said: "I would be surprised if they didn't go to international firms for other forms of assistance."
He said he believed the refusal by South Africa's law firms to assist the Guptas was based on commercial pressure on the firms by both shareholders and "other clients".
Both ENSAfrica and Webber Wentzel said they could not disclose who they acted for, or who they were approached to act for. Both firms confirmed that the Guptas and their companies were not their clients.
Hogan Lovells failed to respond to questions.
Although industry experts said the treaty went against international best practices and South Africa's obligations to global financial co-operation agreements, the Presidency insists it is of mutual benefit to both countries.
The treaty stops the South African Revenue Service from taxing high net worth individuals on their global incomes and assets should they become UAE residents simply by obtaining a three-year residency permit.
While it allows SARS to claim a one-off 18% exit tax on South Africans' local assets and income when they leave the country, SARS loses all taxing rights thereafter.
Tax experts said the global wealth of the Guptas and their associates - many of whom also live in South Africa - ran into tens of billions of rands. Last year's Sunday Times Rich List listed Atul Gupta as South Africa's seventh-wealthiest person, with a South African fortune of R7.6-billion.
The treaty has been labelled "unusual" by tax specialists due to tax benefits being obtained simply by acquiring a Dubai residency visa. Several of the Guptas are residents in the UAE. Duduzane Zuma, who owns a Dubai flat, obtained his three-year UAE residency permit in October 2015.