I agree that she should concentrate on the one she likes best. If she likes them both the same, it’s probably true that the oboe offers some additional opportunities, because there just aren’t as many of them. But that has a flip side, especially if she isn’t a super-talented player–if you’re auditioning for a band or orchestra, there may only be one slot for an oboe, while there are multiple flute seats (especially in a band).
The flute may make a better fun instrument for a couple of other reasons–you can play flute in a marching band, but (usually) not an oboe.