"Is she on the way?"
Translation:Er hun på vei?
At first I also wondered why it was "vei" instead of "veien", but then I realized that in spanish we use the same construction: "¿Está en camino?", rather than "¿Está en el camino?". It's not the first time I see norwegian being more similar to spanish than it is to english in this kind of sentences. Another one is when you ask for somebody's profession. For instance, in english you would say: "Is she a teacher?", while neither in spanish nor in norwegian we use the article: "Er hun lærer?" and "¿Es profesora?".
Also like the French expression, also borrowed into English, "en route". No one ever says "en la route" (or, in line with other questions on this subject below, "en sa route") where a simple "en route" will do for this purpose.
Interestingly, route and vei are related through Latin and before! A Roman road or highway could be referred to as a "rupta via", literally a "broken road", broken in the sense of having been opened and constructed by force of labor, sometimes with military action as well. Just the 'rupta' part was (without semantic justification!) eventually morphed into French "route" and Spanish "ruta" and others. Meanwhile, Latin via most likely descended from proto-Indo-European weǵʰ-, whose other descendants include proto-Germanic wegaz, from which came the English 'way', also Danish 'vej' and Norwegian and Swedish 'vei'.
It's a phrase, where vei has taken the meaning of travel and movement (not just the physical road on which you travel).
If this lady has a long journey, and going by road is one of the stretches (as opposed to railway or hiking on a trail), you could ask Er hun på veien? because the actual road is a point she needs to be at at some point and whether or not she has reached the road yet will tell you something.