There is a 3 year statute of limitations on this type of thing.
Marital misconduct has little if nothing to do with the division of marital assets and nothing to do with child support. The only thing that an affair would affect is alimony and I assume that you are not asking for that…in my opinion, do not settle for less that what you are due. 50% of the marital assets and debts and what the guidelines suggest for child support.
If his accusations are not valid then there is no need to worry. Alienation of Affection suits are expensive, time consuming and difficult to prove. Most of the time, from what I have read on this forum, a letter is sent to the 3rd party stating an amount of money necessary to keep this out of court. The decision must then be made of whether or not the amount asked for settlement is less than attorney’s fees would be to defend against this.
Also, you should consult with an attorney for all of this. Mention to your attorney that your stbx is threatening you with this to “persuade” you to settle, and also be straight with the attorney about the “friend” and the relationship. Attorneys work better with all the facts and he/she can tell you where you stand.
If you do decide to settle with your stbx to keep this from happening, I suggest that you have this put into the agreement also. This way, a year from now when you are only getting 1/2 the child support you should be getting and did not get all that you were entitled to in the divorce, he can not still go back and sue the friend.