Provided he wants to cast exactly thirty feet, and likes the action of the rod that manufacturer thought their customer would like.
Not all line manufacturers religiously follow the AFTMA spec. Rio Grand, for example, which used to be a "half weight" line (their 5 weight was in the gap between the 5 and 6 weight specs, for example) is now a full line weight heavier. (A 5 weight is at the lower end of the 6 weight spec.) There's a reason for that. It's because people who can't get their head around the idea that they're allowed to use a different line weight than what it says on the rod, can now buy a line that makes an ultra fast rod cast the way they want it to. And the easiest way for a rod manufacturer to make a rod "faster" is to rate it one line size smaller than most people are comfortable casting. (Fortunately, most makers seemed to have backed off this tendency recently.)
And even if that's what you want -- a ultra fast rod that casts a country mile when you need it to -- if you find yourself fishing on a small stream where the average cast is 10 or so, you'll find it may not load very well at that range. And conversely, if you've got a rod that load nicely up close and personal, you may find it doesn't carry 40 feet of line very well on a false cast before shooting, but it might if you go a line lighter.
The manufacturer's recommendation is just that -- a recommendation. It's a good starting point, but it's not carved in stone.
Addendum -- I'm not slamming manufacturers here. They're responding to real market demands. As an example, when TFO first came out with the Axiom series, they made a 6 weight and an 8 weight, but no 7 weight. Apparently, Lefty Kreh had advised them that there was not need -- the six weight would a seven weight line easily. (It does -- I've got one and use it with a 7 weight). From a technology standpoint, he was correct. From a marketing point, not so much. People demanded a rod that said 7 on it. TFO now makes one in a seven weight. (I think they also had a gap between 8 and 10 weights, now filled.) I can't blame the makers for giving the people what they want, even if what they want isn't necessarily the best choice.