The procedure is to reinfect them in order to get a series of high fevers over a short time period.
You don't get that, apparently, with naturally occuring malarial infection.
Wow -- look what I found:
Impact of vivax malaria on the immune system of HIV-positive subjectsCenter for AIDS Control and Research, Municipal Health and Anti-Epidemic Station of Guangzhou, Guangzhou, P. R. China (XP Chen MD, PhD, BQ Xiao PhD, WJ Shi PhD, HF Xu MD, K Gao BS, JL Rao MD, ZB Zhang MD)
A beneficial interaction of malaria and HIV co-infection has been reported in a hospital-based study and in a well-controlled cohort study in Africa. In our previous studies there was an increase in total trend of CD4 cell counts in HIV-positive subjects during two years follow up to malariotherapy (therapeutic vivax malaria).