Harry was ready to defend his actions, if necessary; why shouldn't he have told Luna to send for help if she needed it? Admittedly, now they were in the strange position of having her with them -- he wasn't sure quite what to do about that. But he couldn't have just left her to the Death Eaters. And in his defense, he had been thinking of just this sort of occasion, in which someone found out Luna wasn't under the control of whatever mind altering spell -- or, if Luna was right, potion -- everyone else was under. Voldemort had killed for much less. But Ron wasn't accusing him of doing something wrong, so he didn't say anything, either.
"You said you thought it was a potion, or a drug," he added, when Luna finished. That was probably the most important information he'd gleaned from this situation. "That they were trying to use to control you. That they're probably using on everyone."