When I play with guitar players, I usually try a couple of things: Key of D (in either DAd or DAA tuning) and Key of G using DGd tuning. D and G are common and comfortable keys to play on guitar. If your guitarist wants to play in C it's easy to capo the guitar up one step (which puts it in D) and you play the dulcimer in D (again using either DAd or DAA tuning).
If I am doing a mix of chord style and noter-drone in the same session, I like to use DAd for chord style and DGd for noter drone because you can quickly re-tune that middle string from A down to G and it's easy to hear the perfect 4th between the D and G strings. It sounds like the first two notes of Here Comes the Bride.
The greatest music is made for love, not for money -- Greg Lake