He's a possible witness. I'm sure Blago would proffer all kinds of goodies on the former pres that left him out to dry.
So works our legal system. Can't tell you the number of jailhouse informant cases I've worked. This is no different, only higher profile. It's messy business, but it makes the world go 'round. Credibility is not a problem if you have foundational documents to back-up the informants story.