My friend has an Android that doesn't have cellular data on it for the moment. But I have an iPhone and I'm not sure if he can still send and receive texts from me if he isn't connected to wifi.
Welcome to the forums. Text messages should go against his text plan, and not count toward data. The exceptions would be if he texts multiple people, or sends any kind of media, pictures, short videos, etc.
Welcome to Android Central! SMS messages can't be sent over wi-fi--they have to be sent over voice signal (unless you're using something like Pushbullet).