When I served on a grand jury we could ask anything we wanted to ask, even dumb questions. That might vary state to state, though, and I didn't serve in TX so maybe someone can clarify this point regarding grand juries in TX.
This of course doesn't negate the fact that this blabbermouth violated the order of the judge to keep his trap shut on all matters pertaining to the case and the fact that grand juries are only presented with one side, the prosecutor's.
In TX the GJ is actually in charge and if it it so desires can instruct the DA what witnesses to provide, issue subpeona's, and ask any questions they wish.