If your fiddler can play the parts that would be sung as vocal harmony and if the two of you can hear (and agree) on the parts that would be harmonized and the parts that would be solo, the two of you will probably start to be able to Hear (presuming you're playing by ear?) her part.

LouieRoy is right, some fiddlers just don't have the knack for doing backup.

If you can get a happy balance, I bet both of you will have a lot of fun with it.