Don't worry about it there is always a Malaria risk in the DR and I already told my guests to get them before I heard about these warnings. The pills are harmless you take one a week for 2 weeks before, the week you are there and 2 weeks after. All my guests have had no issues with it so far they all feel better to be safe than sorry and the perscription is so cheap like $5. We had no issues with the pills making us feel ill and we did drink while we were there and we had no problems. I would just explain to my guests that no matter where you go in the world that you are at risk for many things so why should Malaria in the DR be any different?
Also it is transmitted by mosqutio bites and I never saw 1 when I was there. The resorts are well taken care of and if you do go where there are lots of them like quadding or something put bug spray on.