For reasons of self-interest, most employers are not going to fuck with their exes' reputations after parting ways.
There are exceptions-- sometimes you have to hire legal professionals and very occasionally one might have to hire illegal professionals to make someone do the right thing-- but those are way outside of the norm.
If he does good work and still gets fired, it's probably by a boss who had little choice in the decision and doesn't want to fuck him over, not someone who's going to be vengeful and make a bigger mess than what already exists.
There are exceptions-- sometimes you have to hire legal professionals and very occasionally one might have to hire illegal professionals to make someone do the right thing-- but those are way outside of the norm.
If he does good work and still gets fired, it's probably by a boss who had little choice in the decision and doesn't want to fuck him over, not someone who's going to be vengeful and make a bigger mess than what already exists.