I'm French. She says it because of the wine, but maybe it has to do with his illness? Maybe not...

It reads to me as a dark joke by repeating a common caution that is now completely irrelevant. Note the "Je ris" ("I laughed" for the non-French speakers) immediately afterwards. I'm presuming that's what kome think you missed.

It's meant as a joke. As a wife who is worried about his health, although irrelevant in his current state, continues to pretend having a normal life.

Such sentence is very common in my (Belgian) entourage.

This is also why he adds he is smiling.

Yeah, I totally missed the sarcastic tone. I just thought she was making the joke on his actual illness, not the possibility of having another illness due to wine abuse.