If he enters Thailand on a 30 day stamp, he can likely just exit within the 30 days and then re-enter on the DTV. Don't really see an issue with that. You're meant to apply for the DTV outside of Thailand, which your friend did. Nothing wrong with taking a vacation in the meantime, whilst you wait for the DTV to be approved?
Once you're a tax resident, foreign-sourced income (like salary from a foreign job, dividends, rental income abroad, etc.) becomes taxable if you bring it into Thailand in the same year you earned it.
For example, you earn £50,000 in the UK in 2025 and transfer it to your Thai bank account in December 2025, it is taxable in Thailand.
However, if you earned that same £50,000 in 2024 and only transferred it to Thailand in 2025, it’s not taxable, even if you’re a tax resident in 2025. Thailand taxes foreign income only if remitted in the same year it's earned.
The ask:thailand community, consisting of multiple Q/A groups with over 100,000 members, powers this platform. It is not an official government resource. Our members actively contribute to this resource, and while we strive for accuracy, we cannot guarantee its complete reliability. Assistance to travelers is provided as a community service.