It would be so nice if those people could have access to a coctail of synthetic antibodies that gave some protection to covid-19, the flu and others. Or perhaps a generic antibody that protects against most viruses(unlikely, given such an atibody does not exist in nature).

Antibodies don't themselves help fight infections; their role is simply to identify dangerous pathogens to white blood cells. In immunosuppressed patients, those cells often aren't present (or aren't responding properly), so adding antibodies wouldn't help.

Antibodies also initiate the complement cascade and can thus kill cells directly.