In the State of Florida, mediations are confidential negotiations, as well as most, if not all states. The mediator must preserve the process and it is not his right to disclose that which was discussed. Any one of the parties to the conference can forbid the mediator from testifying. Similarly, if all parties give their consent a mediator can testify.There are some exceptions to this rule, that might vary from state to state. As an example, if it is discovered that crimes are or will be committed, domestic abuse, etc.
A mediator, like anyone, can be subpoenaed to testify at a hearing or trial. There are many valid reasons for a mediator to testify, as briefly mentioned above.
The appropriate remedy for the mediator is to seek a "protective order" when served with the subpoena. Please remember that a subpoena is a court order and as such cannot be ignored. In essence, you are asking the court to "protect you" or excusing you from testifying or in the alternative have the presiding court official limit the nature and scope of your testimony. This is done to limit the questions that will be permitted and the scope of the questions. The Judge would have great latitude in that he might have the mediator testify "in camera" or outside of the presence of the public or the parties, limit his testimony to specifics, etc.
In the fact pattern above, the deputy sheriff, who was not a party to the mediation, was an observer to the conduct. It would seem that spouse could reply upon the observations of the sheriff to prove their points with the courts.