The astronaut in question may choose to disclose that they had the medical emergency and possibly its nature, but it seems wholly reasonable to not single them out (when it affects the whole mission) or disclose their medical status.
Disagree; this is completely taxpayer funded and we deserve to know every detail relevant to mission status. In this scenario knowing what illness and why it's grounds for a return is very relevant. That said, I can see NASA delaying information release to figure out a good strategy for it while still respecting any wishes of the sick astronaut with regards to disclosure.
Especially since every movement up and down from space is expensive and risks the life of the crew, it'd be a bad idea for NASA to name the astronaut ahead of time.