The ring must not be moving during engraving, and this might be the reason why he is getting uneven engraving on the ring. I believed that if he is using the engraver correctly, he should not encountered such issues. For the self-centering jaw, there is a knob for users to unlock and change the groove.
Can you direct him to the video on our website, http://www.presidium.com.sg/e-tutorials.html (Presidium Inside Ring Engraving Machine)? We do have product video that are useful in guiding the users to use the product correctly.