He has returned.
Even if TR did get a Z-Ring, I'm sure Tapu Koko would immediately take it away from their hands. Don't those are given to people with good intentions?
Or the Dark Z Crystal breaks after it's first usage, but I don't know about that.
they could find a way around it, I mentioned this as an idea a while back, but if Tapu Koko doesn't let them have a proper Z-ring, they could send data of the Z-crystals back to their HQ and get sent "tech" based Z-rings(basically Z-rings that are machines) that allow them to use Z-moves without having to get a normal Z-ring, and you can't say it's out there since TR has been getting more tech from their HQ over the past few gens.
I still wonder how Kiawe's going to act when they next encounter TR, cause he's bound to go Inferno Overkill mode knowing that evil thieves have gotten their hands on a Z-crystal.