Your question is not completely clear. Is the person you don't want to testify against your co-defendant? Did the State give you immunity in exchange for your testimony against this co-defendant? If that is the situation and you do not testify, I am sure the State will not grant you immunity as you are in breach of your agreement with them. You should consult with an attorney before you do anything, as it sounds like the repercussions will be serious.
You have a complicated and potentially very serious situation. If the immunity offer is adequate then it will override your fifth amendment privilege. Failure to testify could be punished as contempt of court. Also, you will have violated your agreement with the Government and can expect that the charges against you will be reinstated and prosecuted. I think you had better get an attorney on board to work this situation through.
What can the punishment be for contempt if you refuse to testify? That might vary widely depending on the jurisdiction in which you are prosecuted. I mention this because of a recent case from the Illinois Supreme Court in which a witness who refused to testify in a murder trial after the judge ruled his fifth amendment claim invalid was sentenced to TWENTY YEARS FOR CONTEMPT OF COURT. The Illinois Supreme Court found the sentence excessive on the facts of the case, but it did so by a divided vote and the witness escaped a twenty-year sentence by a hair's breadth. Tread carefully here, and have a good and knowledgeable attorney at your side.
I agree with my colleagues, this is a very serious situations, especially if the State agreed to reduce or drop charges against you in exchange for your testimony. I would strongly advise you to immediately consult with a defense attorney because there will likely be very, very serious consequences for your refusal to testimony. I would make sure you do this immediately, as you do not want to try and deal with this situation the morning of the trial.
This answer does not, nor is it intended to, create an attorney-client relationship or constitute attorney advertising. Rather, it is offered solely for informational purposes. The facts of each case are different and unique, it is critical to consult with qualified counsel with whom information can be shared and assessed under attorney-client privilege, so that competent and quality advice can be obtained on which you can make informed decisions
Speak to an attorney. But TECHNICALLY, Florida Statute 914.40 requires you to testify against anybody when subpoenaed by the State. I say technically, because the what is technically required is different from what is practically required, which is why you need an attorney.
If you are under subpoena you need to at least appear and go to court. You cannot simply ignore the subpoena because its a court order and violating that in and of itself can cause you a lot of problems. Once you are at court, tell the judge you want to assert your fifth amendment rights and if you do not already have a lawyer, ask for one. At that point your lawyer can help you sort this out. Usually if you are granted immunity that overrides any fifth amendment privilege and you can be compelled to testify. If you refuse to testify at that point, you can be held in contempt, in jail, at least until the trial is over. However, if you already have a lawyer you need to talk with him/her about this. And if you were charged and the charges were dropped in exchange for immunity and your testimony, then either you fulfill your end of the bargain or all bets are off and you can be charged again. If you go this route, this could be especially problematic if your proffered or debriefed in exchange for the immunity and told the government everything because those statements you made can be used against you in the government's rebuttal case. Your ability to defend at trial under these circumstances is compromised to say the least. You definitely need to consult a lawyer on this one.