I just posted these photos and text showing the difference between the three Code 55 N-scale track products that are presently available.
But, I'll post 'em again here so they're easily accessible for readers who are interested in this topic.
First, tie size and tie spacing for both Atlas55 and ME Code55 flex are slightly different...just slightly...hardly noticeable, and since tie sizes on prototype turnouts were/are also mostly different than just the track they were/are attached to, nobody is going to see any discrepancy, or "oddness" between Atlas55 flex and ME #6 Code55 turnouts.
The main difference between Atlas55 and ME Code55 flex is what attaches the rails to the ties. ME Code55 has tie plates and relatively small spikeheads, and Atlas55 has tie plates and big blobs that interfere with pizza cutter flanges.
You may be tempted to say that nobody is going to notice the difference between ME Code55's spikeheads and Atlas55's big blobs, but, I can assure you that people notice. Also, any pizza cutter flanged car or engine will notice too and may not be able to run on Atlas55 N-scale track products.
From a functionality/installation standpoint, Atlas55 flex is "floppy"...floppy flex, just like their Code 80 flex. The track wants to assume its own curves, and is difficult to get to adhere to a centerline if you want varying radii or spiral easements. Also, the loose rail weakens the structure of the flex and will pop loose much easier than a flex that grips both rails securely.
Micro Engineering Code55 flex is a different experience from Atlas55 to install, since it is stiff. This means that smooth curves are more difficult to achieve, as well as straights...but not THAT difficult once you get the hang of it. Also ME flex's stiffness will cause the ties to space out and bunch up, but are easy enough to slide into their correct places. However, ME flex will definitely hold to a drawn center line, or to the line between two pieces of cork roadbed. Using the old M1A Eyeball method makes smoothing the curves pretty easy, and a thin steel straightedge will make your straights straight without much effort.
Photo (1) - Comparison between Atlas55 flex on the left and Micro Engineering Code55 on the right: 
Photo (2) - Comparison between Atlas55 #7 turnout on the bottom and an old Micro Engineering Code55 #6 turnout on the top:
The ME Code55 #6 turnout is extremely close to prototype spec's as far as tie size and spacing is concerned. It deviates somewhat in the length of the frog and the length of the closure point rails, as well as spacing for wide N-scale flanges everywhere.
As you can see from the photo, the Atlas55 #7 is virtually the same length as the ME #6, but should be much longer than a #6. Atlas55 does this shortening on all of their turnouts so that they fit when using their Code55 sectional track.
If you can design your friend's layout using nothing but ME Code 55 #6's, that would be good since they are the most prototypical looking, and have those cast nickel silver frog & guardrails, and nickel silver closure point rails. Also the over-center spring mechanism at the throwbar negates the "need" for a switch motor of some kind.
Since you're gonna use the ME turnouts, why not use ME flex? If you decide to do that, make sure you get the unweathered version which is much easier to solder and bend. Also get ME's Code55 rail joiners.
Although the ME flex may be more difficult to install, the end result will be well worth it, plus knowing you've put down the best-looking N-scale trackage available commercially, especially after you've painted, weathered and ballasted it.
If you'd like to know how to make rail joints invisible I'll be happy to show you how I do that.
Cheerio!
Bob Gilmore
