In general, people in Africa probably have a lot more understanding of HIV/AIDS and the testing procedures than people in the West do. For example:
You say that HIV testing requires drawing blood and testing at a lab. Nope - HIV testing can now be done with saliva, in a cheap ($40) device, and give you results in about 15 minutes. A person in a country with a high rate of infection would very likely be at least somewhat familiar with such a thing considering that those tests are vastly cheaper than the old blood tests. Moreover, as far back as 2015, there was a smartphone dongle that came out that can test for HIV.
Given that we have gone from tests that required a doctor visit, could take days for results, and required blood to tests that can be done anywhere, require a little spit, and 15 minutes to get results, it absolutely wouldn't surprise me that someone would think that advances made it so that perhaps sweat from a fingertip could be analyzed to give a diagnosis. Given that we have gone from gigantic, hideously expensive satellite phones that didn't do anything but allow you to (barely) make and receive calls, to tiny, dirt cheap, and ridiculously capable smart phones, it absolutely wouldn't surprise me that someone might think that a phone could now be capable of performing that analysis.
And finally, she probably knows many, many, many people who have HIV or have died from AIDS. In many countries in Africa, HIV/AIDS is a common, everyday thing that many people deal with, either as having it or having people close to them who have it. To say someone is stupid because they allow a prank app that's just shy of feasible and that preys on an omnipresent threat - that's not just ignorant, but cruel.