I think you know the answer to your second question already....since you are liable for anything that happens to your rental car if it is rented in your name (I am guessing), why would you let an unlicensed driver drive a car that is your responsibilty in ANY country???
It is pretty mucha straight road all the way through Juan Dolio, San Pedro, La Romana, Bayahibe and then follow the road signs to Bavaro once you reach Higuey. Wouldnt your Dominican "friend" know the way?
I would recommend taking the bus, save yourself the hassle and the $$$!