Should he wait for the Titan Black Edition?
I'd say I think so. Even the 790 is what I'm thinking.
790 I think would be ideal since it is meant to feature 5GB of usable video memory with the power of two 780's behind it. The best option if he can be a little patient and return the 690 now. He did just buy it after all so returning it shouldn't be difficult.
Good call on the 790 or Titan black, hadn't thought about those.