I do not think I would do that.
I was in
Dajabon last week.
The road between Montecristi and Dajabon is in terrible shape according to local sources, and the buses use a route that goes through Mao->Santiago Rodriguez->Los Almacigos->
Dajabon.
There are at least six army check points that require you to stop and be searched for illegal Haitians. If you miss one of them, they have huge nailed boards to trhow in the road and ruin your day.
Better make arrangements for your friend to meet you in Santiago, at the Caribe Tour station. Then you can take a trip to Sosua or any other beach.
You trip sounds nice, but unless you know this country, I would not advise it.
In fact, unless you have driven here a lot, I do not advise renting a vehicle, but rather make arrangements to rent a car and driver or use public transportation and local taxis. For a newbie, it is just as fast, a lot more fun and a whole heck of a lot safer and hassle free..
think about it.
HB