It is common to obtain character and rehabilitation letters for a service-member at trial. There is an art to getting and using good letters.
Rule 1, don’t have a letter (or testimony) in which the writer impeaches the verdict. “This is out of character,” or something to that effect is much better and won’t draw criticism from the judge.
Here is a link to Simple Justice, a blog worth following, in which there is a discussion of support letters.