Looks like John Good was the second instance of defense witness violating sequestration:

Zimmerman’s friend, financial supporter and key witness sat in Court before his own testimony. He then told the jury he loved Zimmerman like a son, spent about $1,700 in clothes so that he could show “respect for the process” and also wrote a check for $2,500 and contributed $500 to Zimmerman’s defense. Despite experts saying you need close familial relationship for voice identification, Donnelly testified, that, “There’s absolutely no doubt in my mind that it’s George Zimmerman.” After his testimony, prosecutors claimed unfair surprise and asked Judge Debra Nelson to exclude his statements. Prosecutor Richard Mantei said the defense failed to inform the prosecution that Donnelly was going to testify on the 911 call. During his deposition, he denied having listened to the recording. The judge denied striking his testimony.

Evidence reveals he sat in Court. Not only that, but he sat in Court during crucial witness testimony. The defense usually has the obligation to make sure its witnesses follow the rule. The State should have reported it and made it an issue for the judge. Bailiffs and court personnel are also tuned into these concerns. The ball was dropped. The question is was this a violation and what is the remedy?

Was this a violation?
The extent of the violation depends on what the “evidence” shows- what he saw, what he heard. Sometimes there is no harm from a witness hearing attorney argument or even other witness testimony. Some error is harmless. However, it appears that Donnelly attended crucial witness testimony.