I don't want to diminish the severity of the situation, but typically a person newly diagnosed with HIV can be severely ill (fever, weak etc), but won't be withered to a skeleton so this part is a bit of hyperbole. If they were that frail they wouldn't have been able to sign on for duty in London.
I wondered about this too... but the OP doesn't say how long the FA spent in hospital... it could have been weeks. The OP also doesn't say this was newly acquired, it's possible the FA could have been HIV positive for many years and been undiagnosed. If this hospitalisation was after many years it could well have been serious, with a long stay.
Hence my post that this may have complications beyond the FA's termination... there could be a parter(s) also at risk.