No. For one, ball machines are noisy and distracting to others around him. Balls are bound to be everywhere, even on other's courts. However, if it's a private club and the person reserves the court, I'd say yes, he should be allowed to use it as long as it doesn't distract others. Some facilities have a separate court for this purpose. As for a public court, if someone pulled out a ball machine next to me, I'd be smacking balls at him while he's trying to groundstroke. I'd have every used ball in my bag out, and when my playing partner hit it to me, I'd aim everyone at the guy using the machine, hoping he'd get the message.