Legal Question in Business Law in California

It depends somewhat on the nature of the confidentiality agreement. If the confidentiality agreement concerns your employment with the same employer being sued, most likely it would apply and would preclude you from being a witness against the company. (That may be the purpose of a confidentiality agreement in the employment context, especially if you were terminated and given severance in exchange for signing it.) If the confidentiality agreement applies to patents or other intellectual property that are not the subject of this lawsuit, then you may be able to testify as a witness. You should read over the confidentiality agreement carefully to see exactly what it says about what is considered confidential information. If you are merely being called as a character witness or to describe public interactions with this colleague or treatment by supervisors, you might (or might not) be okay.

And, yes, if you violate the terms of the confidentiality agreement, you could be sued and may be liable for paying your opponent's legal costs as well as your own. That is in addition to any damages the other party to the confidentiality agreement can show.

It would be best if you have your own attorney (not one representing your colleague) review the confidentiality agreement and advise you. Likewise, if you are subpoenaed as a witness, you should engage an attorney to advise/protect you. You could ask that the colleague you are helping pay for any attorney fees you incur in doing this. (In addition, witnesses are entitled to statutory daily fees and transportation costs from the party subpoenaing you, but often they aren't paid unless you ask for them.)