The reason for this man's mouth becoming dry is because he is anxious or nervous about seeing the girl.
His mouth could either be actually dry, or it could just feel dry even though its not.
It could be becoming dry due to some reactions of the anxiety he is feeling. The most common cause is from breathing primarily from the mouth.
When someone gets anxious they may breathe heavier than they would usually do, and often just through the mouth and not through the nose. It is this constant air, in and out of the mouth, that will cause it to dry out.
It could be also due to the bodies fight or flight response. When the man see's the girl, because he doesn't want to see her, the body will prepare to either confront the girl or run away.
When this happens the body may take liquids in the body and move them around to where they are needed more. In this example, the saliva may be removed from the mouth.
Less likely is that the dry mouth is caused simply by dehydration. When someone has not drunk enough, then they are prone to worse anxiety.
There is a simple cure to this symptom, and that is just to drink some water to rehydrate the mouth.