Using a 9 Axis Sensor

So I know someone who has a problem plucking their hair, and I want to help them by making a wristband that vibrates whenever it gets close to their head. Problem is, I do not know what sensor I should use.

I am thinking about using a gyroscope and magnetometer duo to sense the orientation of their hand. When N is facing upwards and the gyroscope is vertical in the Z axis, then it will vibrate and indicate that their hand is at their hairline.

Would this be an efficient approach or is there another sensor that could be used? I was also thinking about an ultrasonic sensor to detect when anything got close to their head, or maybe even dual sensors that sense when they are close to each other.

What do you all think?

6-DoF IMU is what you need. Magnetometer gives only yaw-correction which is unneeded for this.

IMU give the orientation (with random yaw you can ignore), you just want to pick out the angle of
arm, if its too close to vertical upwards, the arm is raised.

The extra 3 axes are not a problem, just ignore them!
3-axis accelerometer + 3-axis gyro = 6DoF IMU. For a simple life find one that outputs
quarternions, DCMs or Euler angles directly to save you having to calculate them.