How does Ruby help Robicheaux in this section?

Robicheaux takes Ruby to a motel, in the hope that Ruby will identify Harpo Scruggs as one of Jack Flynn's kidnappers. However, Ruby recognizes Alex Guidry as the man who showed her mug shots of people he said he had killed the night Jack Flynn was murdered. Ruby also places Harpo Scruggs and Alex Guidry in the same diner where she had gone with her friend Connie.