Co worker fired 2 days after being named as witness in my retaliation complaint

I had filed a grievance claiming I was being retaliated against for getting union backing for a wage dispute. I was suspsended for 3 days amongst other things. A co-wprker had come forward and told me of a damaging conversation he had with the superviser in question. Two days after naming him as the witness he is fired for supposedly changing the date on his doctors note. When I had the "investigation" into my complaint the HR manager said the employee was fired so his statement is invalid and dismissed my concerns and closed the investigation. There is more to it, but these are the bones of the situation. No one wanted to step up to help me after the employee was fired, and I feel terrible because I think his termination was just to "off a witness" so to speak. Is this even worth trying to get help with?