diamond purchasing should be done with research. make sure your friend doesnt jump at the first diamond he sees.
if he is just getting a diamond for 7-8K, he will most likely be able to get a very nice 1.25Karat F-H, SI1/2 diamond. round cut is the most expensive, so it depends on the shape too.
if he wants a 1.4, he can settle for a lower color rating as well as clarity rating. this does not factor in the mounting prices, which will depend on whether it is a solitaire or if it comes with baguettes on the sides and whatnot.
lots to think about, but diamonds are probably the most marked up items on this planet. if he can find a friend who works in the business, he can get a significant discount as compared to the raping the stores want to give him.
got anymore questions just let me know, i've been looking at diamonds for my GF for the past 6 months. not easy finding a good deal, so take your time.