Once you get the DTV you will NOT have any issue to enter Thailand. you can even apply tourist Visa from embassy and go to Thailand without any issue (i have done it before i got the DTV), they only banned you for using visa exemption too many time in one year (dont argue with me about there is no mentioned the limit anywhere, i am just telling you what i have been told by the Thai IMMIG), i am sure if you wait another 6-8 months outside Thailand then you can even come with visa exemption....
i myself also thinking to get Indonesia’s digital nomad visa, always have a backup plan and no harm to have another visa if you can......good idea in my opinion
you might want to think about getting DTV first, if you are flexible and have everything to apply then do not take chance.... before DTV i was told the 3rd time on exempt visa as an American not to come without having proper visa, i was lucky that the supervisor was kind enough still allowed me to enter Thailand to celebrate new year......do not take unnecessary risk if you can avoid,,,
hmm, why he had two passport from same country at the same time valid? if he made new passport after damage the old one then keep both passport and he should be fine....