If his attorney is going to file the decree then there shouldn't be a problem. I can't imagine his attorney telling him to call this old friend of yours to court and frankly I can't see how she would know if you had an affair with someone else as that would be hearsay evidence and not permitted. This really seems to be a bunch of idle threats that no lawyer would put before the court.
I can't see you having to call some guy or guys in to prove you didn't have an affair if she can't directly testify to the fact. Also I hope your husband is prepared to pay a lot of attorney fees to do all of this. Most attorneys would charge an extra $2000- 3000 to change a non contested divorce into a contested action so when he sees the costs of this folly he may be talking a different tune. I still can't believe anyone would testify under oath and commit a felony just over some marital quarrel where there was no benefit to them.
I would just do nothing and see what he does. There's really nothing you can do at this point and if the divorce gets filed it will be over and you can get on with your life.
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