Kouyate (on Brian's picture) plays the ngoni ba (4 strings, 2 outside ones are drones, 2 middle ones fretted).
I asked around about the donson ngoni, it's an ngoni too but more similar to the kora in that the strings are only plucked by thumb and index finger using both hands and there's no fretting.
However it's not the instrument on the link given by the OP. Westaf instrument names can be confusing
I assume he/she's asking about the donson ngoni and not the instrument on the picture?
Anyhow, I found a website that explains it better than I can and that gives some info about tuning for the donson ngoni and a few other instruments in the same family. Probably the bridge placement can be derived from that and from the picture, as these instruments don't have tuners and it's very much a question of string length and placement. Hope it helps!
http://www.kora-music.com/d/familie.htm
"The instrument has 6 strings in two parallel planes with pentatonic tuning: C-D-F-G-Bb-c
right hand: C-F-Bb, left hand: D-G-c (absolute pitch heights vary of course, right and left hand may be reversed)."