As Doomedspeed said, the second hand market is going to be required here if the buyer wants decent gear for CAD use.
The issue is that a budget gaming machine is not in the same league as a budget CAD workstation. And a CAD workstation is not the same, necessarily, as a CAD render node - workstations usually call for better graphics cards and larger drives to carry huge project files and resources (texture and model libraries for example). It's really important to find out exactly what the person will be using Revit for - simple models? Detailed models? Loads of xrefs? Rendering? Animation work? These all determine where he should focus the money on.
Again, a budget CAD workstation is not in the same league as a budget gaming machine, and going too cheap on the components for CAD, just as in gaming, can backfire, but worse than in gaming. If you buy a graphics card for CAD use that has, say 2GB VRAM, but your scenes are loaded with massive textures and tonnes of geometry, you can find that your viewports will simply not display anything if you go over the 2GB limit. I do this on a daily basis with relatively simple scenes and multiple instances of 3ds max, so 4GB VRAM is absolutely minimum.
I would strongly suggest that he look in detail at what he's going to be doing in Revit and think about investing in something that will last for a good long while rather than going cheap and then having to fork out for new gear in a year or less because suddenly he's unable to even work with complex models or renders are taking forever.
The same with the monitor - you really need a decent amount of workspace for CAD work. 27" ideal, 24" minimum I would say. These can be had cheap as chips nowadays second hand and he probably won't need to worry about top-end colour if he's just in Revit anyway, so there are savings to be had there.
If he is going to be using this machine for commercial work eventually then this should be seen as an investment, and worth spending a bit more cash on as a result.
Just my opinions.
