Sometimes I just play a low d note if a b is required, so that it harmonises on the third. If that doesn't fit with the music try a fifth above, i.e. an f, or the octave b above, as others have suggested. You might have to fiddle (wicked grin, as she slips a dreadful pun into the thread) about with what works best, but I find that the third usually works well.
My sister will often play the whole tune an octave above and it makes it more like an arrangement decision - because it changes the feel of the music, but if you don't want to feature the flute over the fiddle doing this may make it too obvious.
Experiment around. Maybe tape the fiddles playing these tunes so that you can play around with them at home and work something out. Or do what I like doing and get the midi of the tune and play around with variations on a music notation programme like Noteworthy so that you can hear the overall effects of each idea.