I think my inmate's attorney messed up the plea agreement, is there anything I can do?

There is only one option. It is called a Motion 2255 which is also known as "ineffectiveness of council". This is a steep hurdle for the defendant to prove. Your description of the events does not make it clear that the attorney dropped the ball, as there are so many variables that are plausible in this scenario but if you have the money and wish to explore your options, you need to find a GREAT post-conviction attorney. Finding an attorney is easy (they will all take your money), finding one that is worth the money and can actually prevail is the tough part. Ask to see their "real track record". Good Luck!