My son's ex atty withdrew from there divorce case, can she now come back ?

No unless your son requests it.
Example. They stop representing your son for non payment. Later he pays things off and then wants to retain the same one. That is OK.

If there is another reason please try to explain it.

My son's ex wife's atty withdrew, now his ex is saying that her atty is coming back after the judge signed the letter of withdrawl, my question is can she come back after a judge signed the letter.

OK I'll ask another way, my son's ex wife's atty withdrew, now he ex is saying that her atty is coming back, my question is, when a Judge signs the letter of withdrawl can they come back?

Yes, they can come back. The attorney merely refiles a Notice of Appearance representing the Plaintiff or Defendant whichever is the case. Happens all the time. Attorneys can formally withdraw and are legally relieved from their legal duties as Officers of the Court when the Judge signs the Order of Withdrawl. Then the attorney can turn around at any time in the future and file the Notice of Appearance with no reason being given.