Your vet is smoking drugs if he thinks your turtle needs another turtle.
A box turtle especially is a solitary animal, only seeking out another in order to mate.
Living indoors is not a good long term way to keep a box turtle healthy. I've been trying different ways for over twenty years, I breed them successfully every year now, and I still can't keep them healthy indoors for longer than about 2 years, they just fade away.
Crowding box turtles is a quick way to end up with stressed or sick animals, so is mixing turtles that haven't been quaranteened. I limit my groups to five turtles in 64 square foot pens, and that's still too crowded.