I gather from the information given that this particular test was in Texas. I'm familiar with Texas law regarding this (having taught in public schools here for many years).
It wouldn't matter if he tweeted the picture from the testing room or waited until he got home to do so; taking a picture of the testing materials in any shape, form, or fashion violated Texas laws regarding the testing and invalidated his test results. In other words, he would have automatically failed. The teacher proctoring the exam (and Texas law mandates that the proctors be licensed teachers) likely got sanctioned by the Texas Education Agency which could have included having his/her teaching certificate pulled for the kid having a cell phone in his possession during testing. The state of Texas does not play about these matters.
Paraprofessionals are now allowed to administer annual state testing (STAAR) -- there simply weren't enough certified teachers to cover all the rooms needed for various accomodations for two different forms of the test each day (individual or small group, oral, signed, etc.)
Everything else you said was spot on though! In addition to invalidating his own test, it's also possible that the tests of other students who received the tweet could have been invalidated. (Since this was a field test, fortunately, that's a moot point.) But since it was a field test, any questions he photographed/tweeted would certainly be thrown out and never used -- possibly costing the state of Texas additional money to replace them. Testing is not cheap; in Texas, it's a billion dollar industry.
IMO, this student behaved very childishly. Yes, field testing is a royal pain and extra work for both staff and students alike -- but it's a necessary step in providing future test questions that are valid and fair, a purpose of which he was in apparent ignorance. (Surprising for a high school student not to have realized this.) There are certainly better ways to protest the requirement of field testing and the perceived misuse of his time than the one he chose.
If this were my kid, I can assure you I wouldn't be defending him, as his mother did. He'd be apologizing to the teacher whose career he put at risk, and he'd be absolutely grounded until he understood that actions have consequences and "protesting" is not a license to hurt others.