She was played by Moon Bloodgood.
Hayley is a member of a Chinese gang who was targeting Monk, who witnessed one of their crimes and ended up in witness protection. In order to get to Monk, Hayley posed as the girlfriend of Randy Disher, with part of her ruse including her and her fellow gang members composing fortunes in specially made fortune cookies for Randy. The first one included being saved by Hayley after Randy is run off the road by a gang member (unbeknownst to Randy). Hayley's ties to the gang are revealed after Randy drives off to where Monk is, after reading a fortune that suggested that he's in trouble. It is assumed that Hayley was arrested for her role in targeting Monk, as Randy fretfully stated that he had to arrest Hayley for conspiracy to commit murder.