Other mistake: When Brass is asking if a witness or someone they are speaking with knows the victim, you can see the victim's name on the bottom of the crime scene photo before brass is supposed to know the identity of the victim.
Continuity mistake: In the scene where Riley Adams takes a victim's wallet from David Phillips, she takes the wallet with her left hand with no glove on. Next shot Riley has gloves on both hands.
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.