I think that in some circumstances being friends is enough to recuse one's self when being in charge of an investigation. For example, IF the police chief in Sanford, FL was best friends with George Zimmerman AND he was either in charge of or BOSS of the people doing the investigation on Zimmerman, then he should recuse himself. Perhaps, he could hire a third-party or give the job to one of his lieutenants. In Washington, DC, I think it's much harder to find independent people since you've got the partisan republicans and democrats who all know each other. If Lynch were handling the investigation herself, then I'd say she should recuse herself.
Now, that was then and this is now. Just recently, Lynch and Bill Clinton fucked up. They met privately on her plane/in an airport for 30 minutes. That's like George Zimmerman's father meeting secretly with the police chief in Sanford FL for 30 minutes (if they were good friends). So, that's the straw that broke the camel's back for me--i.e. it's just enough to say Lynch has to find some way to back out of this. I don't know what that means, but she has to distance her authority over it by making a change in the authority structure or handing it someone who does not ultimately report to her.