In general, a judge in a criminal jury trial can add jury instructions to fit the evidence if the added instruction regards a "lessor included offense". Manslaughter is a lessor included offense of homicide, in general. There are exceptions.
In order to determine if there was a legal error, it would be necessary to see the record. We'd like to see if the judge just added the instruction, sua sponte (alone), or if one side or ther other requested it, and the judge approved it. I see you said the jury requested it, but that seems a bit simplified. They jury may have been stuck, and then the prosecutor made a motion for that instruction, perhaps. I would also look to see if there was an agreement between the parties for the instruction. If the defense attorney did not object, or actually agreed, there may have been a waiver of that possible legal error.
Is this some that could result in a retrial? Yes, if there was a wrongful addition of a jury instruction that resulted in a conviction, instead of an acquittal, then a new trial is in order, as they say in appeals courts.
Does he still have a right to appeal this issue? I cannot tell. Probably not if he already had an appeal without this issue, but it depends on the type of appeal it was.
He probably still has a right to a last resort right to a Federal Court Petition for Writ of Habeous Corpus on that issue as a denial of his 5th and 14th Amendment Rights under the US Constitution to Due Process Under the law, which would seem to necessitate clear warning and notice of all jury instruction PRIOR TO THE START OF TRIAL AND OPENING ARGUMENTS, right?
The defense attorney probably should have objected because it was clearly a trigger to get a conviction out of a hung jury. A hung jury is a win for the defendant in the United States. That jury instruction gave the hold out jurors a way to convict without losing sleep even though they clearly had doubt, right?
What if they would have given the jury a resisting arrest instruction too?????? They would have convicted on that as well. How about an instruction on being a complete A**hole, and generally bad person...??
Here is what an appeal attorney would like the Defense Attorney to have said on the record when that instruction was added:
"Objection! It is a reversible legal error to use that jury instruction at this point in the trial. The trial is over. This is a clear legal error, your honor. You cannot surprise the defense on your own with new jury instructions that have not been litigated prior to trial, tested prior to trial, and noticed on the defense prior to try. We missed the chance to challenge evidence that may be relevant to that instruction because it was not part of this case. This trial is over. This instruction is simply a prosecution oriented way to get a hung jury to convict, to kill the legal and reasonable doubt one or more jurors clearly have. It is a cheap way to give them an out for conviction, when it is clear they cannot vote for conviction now. I want this spot marked with a red crayon on the court's transcript please, and I ask the court for a 10 day continuance for the purpose of seeking a Federal Court order to stop this illegal action via Writ of Habeous Corpus as is Defendants Federal Constitutional Rights under the 5th and 14th Amendments. Objection."
But of course, I was not at the trial, and can only guess about what happened.
In summary, if I were the defense attorney with a case of this sort, getting paid by the hour, I would collect the official record, study it in detail while looking for legal erros, immediately file a notice of appeal if possible, and start researching my Federal Petition for Habeous Corpus relief.
But in the end, it may turn out that it was done by agreement, or that it was legal due to the evidence in the trial, and the fact that it was a lessor included instruction.
In the court record, there may be written briefs on that exact point which was fought over at some point in the pre trial motions.
Hope that helps.