While your question is certainly in the area of criminal defense, I think the better practice area is "Federal Crime" as there are marked differences between state (county) and federal prosecutions and your specific query about §2255 is most definitely a question that pertains to the special rules and procedure in Federal Court. What you describe seems very out of line, but much greater detail would be needed about the Defendant's case. I'm going to go ahead and edit the practice area accordingly. The bottom line is that there needs to be a private consultation with qualified criminal defense counsel with federal expertise. Good luck.
Collateral appeals of federal convictions under 2255 are extremely complex and fact-intensive appeals. The right to bring such a claim arises only upon disposition of the defendant's direct appeals. This is most definitely NOT something that is appropriate over the internet. The time period for bringing a collateral appeal is strictly limited. If this happened to someone you know, the best thing that you can do is retain an experienced FEDERAL appeals attorney for your friend/family member ASAP. The defendant should NOT attempt this pro se.
An attorney would have to sit down with this defendant and review exactly what happened and possibly review the trial transcripts and appellate briefs to determine whether there is a viable or arguable claim under 28 U.S.C. § 2255. Either the sentence would have to be statutorily illegal or in violation of the constitution or otherwise subject to collateral attack. Your post doesn't provide enough information to determine if the sentence qualifies. The essence of your claim as I read you post is that because they charged defendant with a crime resulting from defendant's conduct as an informant which weren't crimes because defendant was acting as their agent, and apparently defendant was convicted of those crimes, defendant was subject to a mandatory sentence which he otherwise would not have been subjected to had they not charged defendant with that conduct. The problem with that proposition is that if the defendant was found guilty of the charges resulting from that conduct, that finding constitutes a rejection of the fact that the conduct was at the behest of the police. The jury's findings relating to facts in the case cannot be challenged under § 2255 at least as I read that section. You should take the documents that you have to a seasoned criminal lawyer to see whether there is a potential Federal Habeas Corpus claim that has a chance of success.
You can always start with ineffective assistance of counsel, but not arguing a theory is not IAC. You really need the evidence you have reviewed. Please consider getting one of the attorneys at the find a lawyer section.