Either Sam Lombardo was responsible for selecting him for the trial (or suggested by his colleague, it's never made all that clear), in which case it's an implausibly major coincidence;
Or Suzie specifically suggested him to Sam, in which case Sam should have been asking more questions about him and questioned his motives, which means the attorney should have played a much bigger role in the proceeding round of backstabs committed by everyone else.
Personal theory: Sam was Suzie's lawyer when she was framed and sent to juvie hall. He probably knew she was innocent and did his best to try and help her beat the charges and that earned him Suzie's trust. Trust enough to bring him into the scheme. As for why Sam would hire him, they imply that Sam doesn't have much choice at the start of the film due to Kelly's family being all mighty powerful as they are. No other lawyer would cross the line to defend him, plus Sam doesn't have much in the way of funds to hire a high caliber lawyer, even if he could find out who would defend the man who was accused of raping a Van Ryan.