Weeelllll, if he waits a bit more, the 2015 model will be even better...
There comes a point when you need to pull the trigger. When that point is depends on each individual's tolerances and budget. There is no one size fits all answer.
Some people like to have the latest and greatest. Some people like to squeeze the most out of their expense. Where do you fall?
If you wait for "the next big thing", you will be waiting forever. It is a good idea to keep your ears to the ground and keep up with the rumors to see if something truly remarkable is on its way, and Apple's announcements are pretty regular. Like buying an iPad 4 now would be crazy.
The rumors for the 2014 model make me appreciate my 13" MBA even more. I am not crazy about a 12" model, retina or not. It will be a little faster, have a little spec bump here and there, but what will determine whether you should get it or not is probably your need to have the latest tech.
Unless you are trying to do something with your system now that is very slow, in which case you are better off with a MBP anyway.....