A question about defamation would
probably best be posted in the "Libel/Slander/Defamation" section of this forum, by the way.
Without knowing for sure who sent the letter to you, your husband (or the co-worker named in the letter) cannot sue anyone for defamation over its contents (if the accusations made in the letter are, in fact, false).
"I believe we know who sent it" is not good enough for a lawsuit.
Telling someone that "what comes around goes around"
could potentially be considered a threat. If there has been some conflict between your husband and this woman in the past, it is possible this woman may have a legitimate fear for her safety (especially if your husband has told this woman he suspects she has revealed private facts about him to you - whether the facts are true or not).
Your husband can wait to see what comes of the meeting next week (and hopefully the matter will all be resolved peacefully then), or your husband can attempt to get a civil harassment order against this woman (to prevent her from further unwanted contact with you or with him), or he could go to an attorney and have the attorney draft a cease-and-desist letter - warning this woman that if she does not stop the harassment, legal action against her may be pursued.
There is little
you can do at this point. It would be up to your husband to take any action.
Good luck.