To solve an answer, we should talk about logical reasoning!

The killer's clothes should be wet, but her boyfriend's clothes are fine and he smells of alcohol, which proves that he has been drinking in the bar. What he said in the quarrel is also true. His business partner confirmed that he was not lying and had reason to answer what happened after he checked in that night. He is wearing pajamas, which is normal, and probably has never left the room.

The most important thing is that no one asked the driver about the clothes. How did he plead first? Only the murderer knows that jumping into the river is wet. He didn't hear the quarrel, probably because he was not in the room. He went out before the girl.

Of the three, the driver was a stranger to the victim. The driver hired temporarily, the girl may not know his name at all, so she can't tell the name of the murderer when she dies. If it's the other two, the girl will give her name.