Another suggestion is to get a capo for her guitar, and she could capo on the 2nd fret, and play C position chords. It will sound in the key of D. When I played guitar with the local dulcimer society, that's what I did. It has a good full sound for the key of D imo.

Ken is right - the chords your wife will need to learn to strum along to your playing would be the same chord names that you are playing on dulcimer. She is most likely to need the guitar chords D, G, A and perhaps Bm. Any simple guitar chord chart will show her the fingering for those chords on a standard tuned guitar.