When I went last August, it was sunny the whole week without any rain. However, it is the rainy season so expects a few drops. As for the humidity, it's very humid at night.
Spoke with my Dominican friend on the weekend and it's very hot (about 40) and humid now - I can only imagine how it will be in August! But no matter the weather, you will enjoy the DR!
Unless your friend lives in Jiman?, or some area near the frontier with Haiti (where deforestation accounts for the higher temperatures), temperatures near 40 are unheard of. The maximum for most of the DR is 35 degrees at mid day.
