For someone to be a good uke takes as much training as to be nage. Its a whole lot more than just a body to practice on who knows when to tap out.
Yeah, that. I think it's sketchy to be practicing techniques on people who haven't been properly taught how to respond to them (and if you're trying to learn the techniques yourself, you're most likely not the one who should be teaching them). Seems like too much injury potential. If he's a karate instructor, learn karate from him.