Are teachers allowed to text message students under 18 years old?

My friend, who is seventeen, texts my high school teacher who is about 45 years old and they have been to lunch on a few occasions. They talk about friendly stuff like what they did over the weekend, people, etc but My friend has a huge crush on him because he's really nice, really funny and, according to her, is "really cute." He is married with two children. Lots of girls have crushes on him and on one occasion, one of these girls sent him a dirty picture. He simply told this girl that he was not at this stage in his life and that was that. He's really easygoing and sends texts to lots of his students. So my question is: Is what my teacher doing inappropriate considering that the student-teacher relationship remains friendly?