sorry this is long. If you want to bypass the bumf, go to the last para.
QofQ: that is also the reason why it is a graduated risk. Antimalarial tabs also cause liver damage, just like malaria - taking it for the short term probably not. For young livers, there is no scientific evidence to say how safe they are.
There are several strains of malaria. some more virulent than others. Africa has some of the more medication resistent strains as well as malaria being more widespread among the population but compared to Sri Lanka, I don't have the stats. For your friend to contract malaria may have been inevitable as the tabs don't always work against the strain you may contract. It also depends on the type of environment your friend stayed at in Zim as well as how long and how much exposure to locals who may have been carriers of the parasite and how many mosquitos he she was exposed to.
To simply say that those precautions don't work is too simplistic. I lived in a malaria country for 20+ years and never contracted it and know many people who haven't. I have not once in my entire life taken malaria tabs and neither do any of my friends. OTOH, I do know people, well off, well educated about malaria ones too, who have had the disease because they have lived for long times in malaria countries.
The mosquito is only the vector. If there is a low incidence of malaria in a region, no matter how many mosquitos capable of carrying malaria is out there, they cannot transmit the disease unless they have a blood feed from a person with the parasite and then feed off you.
Would I take a lo to a malaria country? yes. But I would investigate my options carefully. If I feel like I ought to take malaria tabs to be happy there, then I wouldn't take my lo there. As I don't want to take the tabs so I wouldn't give them to my lo either.