I'd start by checking if your Haitian has his cedula, and a passport. At the office in Puerto Plata I was given a document which listed what an extranjero would need to marry in the DR, but lost said paper a couple of years ago. Sorry but I don't remember the name of the office, but it was right near the main police station near a Sirena. But it did list what two foreigners would need and I assume it would be the same as if two Americans were to decide on an exotic Dominican Wedding.
Der Fish