I was wondering whether it has any metaphorical meaning at all, because in the book, it is term which signifies a hypochondriac. It is used as a joke. So I'm trying to see if there is any connection between frequent flier and hypochondriac, or was this term chosen randomely?
Yes, me too. And you're right. Regarding a translation, when you come across something simple your first guess should be wheather you are missing something! Ha ha
LL - you are too modest! 'Frequent flyer' has developed into an idiom meaning someone who does things often, So if the reference is to a hypochondriac, I suppose they are a 'frequent flyer' in the doctors surgery, i.e. they are always there!