[US] How to help with romance scam if the scammer is too good

A sister is being romance scammed by someone posing as a famous music artist, despite family and police efforts to prove the scammer's identity is fake. The scammer uses convincing methods like phone calls, texts, and video chats, and the victim has already sent money for 'concert tickets and backstage passes'. She is vulnerable due to being disabled, isolated, and going through a divorce.