Who is Sarah's boyfriend?

Hint 1 of 13:

The first clue you can find is in the dressing room.

Hint 2 of 13:

Did you look at the stain on the door?

Hint 3 of 13:

Someone would have to be pretty tall to put a mark that high.

Hint 4 of 13:

Another clue can be found in Sarah's apartment.

Hint 5 of 13:

The rugby sweater in her laundry will tell you where to look for him.

Hint 6 of 13:

He also bought some perfume for his girlfriend.

Hint 7 of 13:

The person who sold it to him may be able to help.

Hint 8 of 13:

Well, if not Belle, try her employee.  (Another question tells you how to
talk to her.)

Hint 9 of 13:

You should learn the brand of cigarettes that he smokes.

Hint 10 of 13:

Did you also know that Sarah went to the opera to watch her sister?