Bass goes in to a Donner splitter pedal, one signal goes to a Korg Pitchblack tuner (so he can mute the bass too), on to a bass amp.
2nd signal goes to a Mooer Tender Octaver to go octave up only, in to a EHX Big Muff, on to a guitar combo.
Started with a Zoom (MS60?) and added to it. Zoom octaver is pants, so tried a sub’n’up, then eventually settled on the Mooer.
The lad has got a good ear and knows what sound he’s after. Coupled with my uncontrollable urge to buy bass-related stuff…. well, I’m doomed.