Is there a certain guitar tab you have in mind, or are you asking for a general concept of this fret on guitar equals that fret on a dulcimer?
Guitar tab is usually lead parts, not chords. The lead may be melody, but more often is an improvised solo part. The real problem with converting it to a dulcimer tab is dulcimers don't have near the range of a guitar. Low parts would have to be moved up an octave to fit on a dulcimer. High parts could not be moved up an octave because the guitar can come up at least as far as the dulcimer. The relocated low parts then will not be quite in relation to their guitar counterparts, but it can be done. It would be best to move a whole section, rather than just certain low notes.
Converting may work, if the solo is not too far outside a certain key. If the guitarist makes a lot of use of out of key chords, the notes of a solo reflect those chords. They have to, to harmonize. For a dulcimer to play this note for note, it may be necessary to use a chromatic dulcimer. CAUTION: OPINIONATED CONTENT! It seems to me it would be better to create a dulcimer solo, rather than try to convert one from guitar. But that's just an opinion, and creating new solos may not be what every player likes to do.
Look for a guitar chart called "Fret board Road Map". It shows what note is at what fret, assuming standard guitar tuning. If the guitarist is using a different tuning, there may be a chart for it, I really don't know. I never bought them. In standard tuning, the 4th string on the guitar is tuned to D, the same D as our bass string if we tune to DAA, or DAD. The 3rd string is G, same as we use for DGD tuning. The second string is B, one tone above our A middle string, and the 1st string is E, one tone above our D melody string in DAD. The tricky stuff starts on the guitars 5th string, where we can't reach any notes below the guitars 5th fret. The 5th is tuned to A, an octave below our middle string. The 6th is E, almost an octave below our bass string. If you have a tab in mind, the starting point is to decide what key it's in, and see if it changes keys or modes. Hopefully, we can find a dulcimer tuning we can use for it.
Paul