Fiddles can be quite difficult to mike because they seem particularly prone to feedback - something to do with the direction you have to point the mike in and the intensity of the sound I think. If using a mike on a stand, I'd agree with what Hiho_Silver said and try and get your fiddler to stay still relative to the mike. I usually put the stand in front to my right, quite high with the boom reaching over my fiddle so that the mike itself is 3 or 4 inches above the bridge. Depending on the fiddle and how directional the mike is, you might want to experiment with pointing the mike at one or other f-hole - probably the one on the bass side. Need to be careful not to whack it with the bow though. If you get feedback problems, may need to move the mike further away from monitors/speakers.

If the fiddler is a permanent addition to the band, they might prefer to get a pick up as it gives a lot more flexibility when it comes to moving about. There's an excellent article here www.judicael.co.uk/articles/violin_pickups.pdf comparing different types and makes of pickup. They vary in price considerably and therefore in sound quality and reliability re: feedback. I've just bought a hypercardioid mike type pick up from Accusound which I am chuffed to bits with (very natural sound and no feedback problems) but not cheap at about £250.