First write a query to find widgets with only one comment:
From here you'll want to bring in the users name from the user table, and count em up again..
COUNT(WIDGET_ID) = 1
One problem with this approach is that if a user comments on the widget more than once, it will be excluded even if someone else hasn't done so. This can be solved by motifiying the starting query.
U.NAME, COUNT(U.NAME) AS NUMBER_OF_WIDGETS
COUNT(USER_ID) = 1) AS SCW
LEFT JOIN USERS U ON SCW.USER_ID = U.USER_ID