I drew out the little junction box is SketchUp, incorporating the headset logo. (I've duplicated and flipped it above just to show the logo, but you only need one box, printed twice.).
I printed two copies, in a colour that matched his headphone cable.
I glued and clamped the two halves together, securing the joint between the round and flat cable. I coloured the logo with a pen. (Some day I'll have dual extruders so I can print in two colours! )
That's one less item on the repair shelf! :)