I would put some super glue in the crack and use a clamp to see if the crack closes up and glues together. Actually I'd try the clamp first and if the wood moved with the clamp then I would remove the clamp, put glue in the crack, and then clamp it. A little accelerator sprayed on the crack after it is glued and clamp will harden it up quickly.
"The dulcimer sings a sweet song."
That's a crack in the headstock, not the fretboard. They are two separate pieces. So there should be no danger of the crack spreading into the actual fretboard.
As Matt describes, carefully add drops of thin superglue to fill up the crack. Then you'll be able to tell if the crack is spreading.
The builder did a wonderful job of putting together a unique instrument. Eight strings on a thin scroll head is a lot of pressure. The sides of the head are perhaps a tad thin for that many strings.
Every instrument has the danger of cracking. An instrument as detailed as yours needs continuous care, especially to maintain constant humidity. It cracked once and could crack again. If you know a luthier nearby, ask them to look at the crack.