You Kill Me - S8-E8
Character mistake: While looking at the evidence that was knocked onto the floor, Greg picks up a woman's shoe in his bare hand causing his finger prints to get on it. When investigating a crime, even one in a "whodunit" game, he should have been wearing gloves.
Answer: He doesn't have the authority to arrest him - when anyone is arrested in the show, a police officer does it. Also he tells Ty that he is going to be submitting his case findings to Ty's insurance company, who would no doubt contact the IRS, who then would have him arrested for fraud etc.