If he knew Ae the woman, she probably left the room with the door open when he went into the shower and signaled them to come in (or possibly just left it open, but not as likely because in person purse snatching is very rare in Thailand, especially in an enclosed space and not via motorbike).
The article said he knew her and he yelled for her and she was in the hall outside the room. That means she was around when he went into the shower.
The police should check her cell phone for recent calls.