"reporting the principal ditribution"

There is no such thing as Principal. Perhaps what you are trying to do is show, per the ordering rules in a Roth, that this taking is Contributions? Have you filled in the 1099-R worksheet to show there is basis (their contributions), and are you sure they are not taking more than this?

You need to input the ROTH basis, should be a button on the 1099R worksheet for that....scroll down near the bottom of page 1