Just leave it attached to the controller that’s moving between devices, the connection to the puck is USB-C so it’s not like you’re moving a bunch of cable and the magnetic attachment is strong enough to not worry about it falling off.
On the other hand, If they have a Steam controller and their GF also has one then they’d have 2 pucks.
I don’t see the problem.
Just leave it attached to the controller that’s moving between devices, the connection to the puck is USB-C so it’s not like you’re moving a bunch of cable and the magnetic attachment is strong enough to not worry about it falling off.
On the other hand, If they have a Steam controller and their GF also has one then they’d have 2 pucks.