What can we do to void the termination letter that my wife signed but forced by her employer?

Typically, employees are asked to sign off on a termination letter to acknowledge receipt of the letter, and not necessarily that you agreed to the allegations.  So if that is the case, there is no need to "void" the letter.  If she signed as agreeing to the allegations, then that does create some potential issues.  She cannot "void" what she signed, but she can send a follow up letter (certified and regular mail) or email making it clear that she did not sign to indicate that she agreed with the allegations.  Rather, she only signed to acknowledge that she received a copy of the letter.  The letter can also state why she disagrees with the claims. In either situation, you should immediately meet with an attorney to review this matter