He'll keep his pledge|| to donate 10,000 mosquito nets to charity|| to help fight Malaria in Africa.
to help fight Malaria in Africa, answers the question: Why is he donating nets to charity? And the answer is: to help fight malaria in Africa. It would then be an adverbial, and describes the entire first infinitive.
There are two bare infinitives of purpose. The second one explains the first one.
A better rendition of it would be:
He'll keep his pledge to donate 10,000 mosquito nets to charity// and// [thereby or that way] help fight Malaria in Africa.
That way there is parallelism in the infinitives of purpose.
infinitive of purpose