The filing makes clear that Coughlin understood how valuable he was to Ring. At one point, emailing in response to Ring's thanks for Coughlin's good work, he responded "Hey man. I have a favor to ask. Any way I can hit you up for Wizards tickets (4) on the 15th and 18th of March?"
"In appreciation for the assistance that Coughlin had provided," the filing states, "[Ring] gave him eight third-row tickets for the Wizards."
As I said below, Coughlin has agreed to fully cooperate with prosecutors, raising the question of what sort of help he might be. Well, for one thing, Ring himself has not yet been charged as part of the investigation. He was a key member of Abramoff's team, not only for his ties to John Ashcroft (for whom he used to work along with Coughlin), but also because he had been a top aide to Rep. John Doolittle (R-CA), who became one of Abramoff's key allies in Congress. The FBI raided Doolittle's Virginia home last year as part of the investigation.
Coughlin worked in the DoJ's Office of Legislative Affairs in 2001 and 2002, then moving over to be the Deputy Director at the Office of Intergovernmental and Public Liaison. He left for a detail with the U.S. Attorney's Office in Eastern Virginia in late 2003 before returning to be the deputy chief of staff of the Justice Department's criminal division. So it appears that the Abramoff investigation would have already launched by the time he got that job (it began in February, 2004). The DoJ has said that Coughlin recused himself from the investigation, though it appears that the revealing emails came as something of a surprise. The Wall Street Journal reported this morning that "Mr. Coughlin resigned as deputy chief of staff in the Justice Department's criminal division a year ago when prosecutors discovered his email correspondence with former Abramoff lobby team member Kevin A. Ring."
Update: Here is the plea agreement struck by Coughlin and prosecutors.