Student or Learner
My friend (wait) ____________ for the prince of the flats to come down before buying a flat.
I totally agree with 5jj and billmcd...
Without further information about the context you've taken this from, 'has been waiting' is the more likely of the two, BUT I think the use of these two tenses in this case depends on this: 1) if your friend has not bought the flat yet, so he's still waiting for the prices to come down, you have to use the present perfect continuous to underline that he began waiting in the past and he still waiting up to this moment, otherwise, if he has already bought it, I think you can use the present perfect simple, because the action started in past and now it's over even though you don't specify when.
I hope this can help you.
Not a native speaker... but only interested in English language.