No it is not. It's breaking the lease contract that Apple and the developer has. The only point where it becomes theft is if Apple requests it back and the 3rd party withholds it.
If I rent a car and do not put a second driver on the contract, but let someone else drive the car (breaking the rental agreement), that person has not committed theft unless the company wants the rental car back and that person does not return it.
While you're right (and I don't believe it is theft), in your scenario if your friend started dismantling the rental car you both would likely be liable for criminal damage.