There are songtaews from Trat to the border every 30 minutes or so. If you are going to drive yourself there are several resorts along that route and also other points of interest. It is a nice part of the world to spend a couple of days exploring. Koh Kong is quite nice for a night or two. And cheap.
I asked this question of an agent in Cambodia early this year , and was told no , it must be a flight . Be aware Phnom Penh is possibly the hardest consulate to get a tourist visa , lately they have a history of accepting applications ,and the fee , and then refusing to grant a visa . It is best to consult a good agent prior to applying .
The tourist visa would have been for 60 days. ? So he could theoretically stay for 150 days with one border run ? What is his option after 150 days ? Do it all again. ?