I think the best way to approach writing such a letter is to state who you are, the nature of your relationship (that you are wife and friend), and to be clear about the amount of time you have known one another, the traits or characteristics you observed about him, and to provide examples of when and how he demonstrated those positive traits. If your husband is "facing a lot of time" as you say, you may want to steer clear of attesting to qualities that are clearly in conflict with his crime so you don't lose all credibility (e.g., don't say your husband is honest and forthright in all things if he has just been convicted of forgery.). I am sure you can find other qualities in your husband that you can instead focus upon. Try to focus on qualities that demonstrate your husband is willing to correct mistakes and learn.
