General Discussion

There's herbs and compounds that hasten recovery or mitigate symptoms, but there's no magical cure, and there's no preventative other than isolation from any disease.

Strong, well fed, well cared for people still died throughout the ages.

Sometimes because their "immune" systems were too good and were too aggressive reacting to the virus, bacteria, or whatever triggered the reaction, attacking the entire body rather than just the pathogen.

Sometimes the weakest, apparently sickliest person survived against all odds because they had earlier been exposed to a similar pathogen, and their body knew how to react to the current disease quickly enough to reduce the worst of the damage from the most dangerous symptoms - like a vaccine would.

Sorry your sister and her son are that far into woo.