I see no action that your ex-employer can take against you for helping out in your colleague's case unless, of course, you are untruthful. Yes, you can be obligated to cooperate further via subpoena; changing your mind has no effect on this.
Your "ex employer" is now no better - and no worse - than any other human in this planet so far as you are concerned. Obviously, life has no guarantees, but you haven't said anything that suggests a vulnerability with respect to that ex-employer. Whether you voluntarily choose to make a statement or not is upto you. But whether you do so or not, you can be subpoenaed. And from whatever you've written thus far, if your former colleague goes to Court, it looks immensely probable that you will be subpoenaed - in other words, yes, you can be "legally forced" to provide truthful testimony. Whether you call that "getting involved" or not, that is the law.
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I agree with the prior responder with an addition. You can also retain a personal attorney to represent you.
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As a general matter, there are protections for people who choose to testify. Under Title VII (the law the EEOC enforces most commonly) there is a clause of the law called the "participation" clause of the "anti-retaliation" provision that basically says the law wants to protect people from adverse consequences for giving testimony. If, for example, you were to testify and tell things that were bad for the company and they then went out and "black balled" you in the market (gave bad references etc.) then that may be protected under the law. That being said, those cases tend not to be very straight forward. The best policy is to tell the truth, don't be vindictive or go out of your way to hurt the company for any of your own personal reasons, but to follow your conscience as far as standing up for any wrongs you may have overseen that affected your coworker.
THIS IS NOT AND SHOULD NOT BE CONSTRUED AS LEGAL ADVICE.
This is not legal advice. Consult with a lawyer about your particular situation.