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.