Don can't just blindly walk upstairs. Maybe the receptionist would help him. ?
Use the bell to summon the receptionist.
You would need to talk to him about something other than flooring or plumbing. ?
Use the Haunter's Mark.
Use the Haunter's Mark on the portrait. You learn about "Olmstein". Talk about that with Serge.
You can call him again with the bell. Try it, it is fun. ?
Talk Serge about Olmstev. And keep calling him back.
Call Serge back 15 times to get an achievement.
At some point Serge says he will believe you if you know the nickname Olmstein gave to Serge.
Call him back, again, and tell the nickname to him.