This would be Dream's friend? Has anyone checked with people in her group of friends and aquaintences? I know it would sound paranoid, but generally they'd have the best idea of who she had been with last, and nine times out of ten, at least ONE of them knows where she is.
The best course of action at this point is to talk to EVERYBODY she knows and spent a good deal of time with. If one of them seems suspiciously out of sorts, start with them in particular. I would NOT want to have this escalate.
I'm staying up tonight, and preparing to go on a little hunt. If she's anywhere on the west side of London, I'll find her. Tomorrow will be the east. This decision is not in any way due to the fact that dreaming is not a pleasant thing, when Morpheus' emotions are high.