Forensic document examiners have testified as expert witnesses in various courts, and other judicial hearings, for many years. The field was recognized by the courts around the world a very long time ago and, in general, FDE testimony is well-received.
However, as with any type of forensic expertise, the decision to permit an expert to testify in a given court proceeding is made by the judge/adjudicator on a case-by-case basis. The Court must decide whether or not to admit expert evidence based upon a number of factors including the qualifications of the expert, the nature of the evidence, the need to have expert evidence presented and so on. There are various legal standards (jurisdiction-specific) that must be met so that an examiner can be qualified as an expert and permitted to give testimony.
Check with the examiner to ensure they can fulfill those requirements.