I think the a lot of it depends on what your friend wants from the games.
Generally though, you can pretty much start anywhere. The only ones I'd recommend against are Meruru/Lulua (because they rely almost entirely on returning characters for their stories) and Totori (because the game doesn't really have a tutorial, and the time limit makes it hard to experiment with game mechanics on your own).
For the ones I explicitly think are the best:
Sophie seems like the best all around choice. It doesn't have time limits that sometimes stress beginners, but still retains some semblance of open world exploration. Plus it's the beginning of the trilogy, so no worries plot wise, and is on of the best games in the franchise overall. It does however, have crafting mechanics that some people find overly complicated (I think they're fine, personally).
Escha & Logy is a fan favorite, and one of the most polished games in the franchise overall. But I'd only recommend it to people that really don't want a challenge, because the level of difficulty is just far too easy. This is the one I started of with, but the lack of difficulty almost put me off from the rest of the franchise.
Totori is the best game of the franchise, and really the only one that actually somewhat lives up to this idea that Atelier games are about time management and being efficient about gathering and crafting. That said, it's bad for beginners because it just throws you in without any real tutorials, has a punishing time limit, and expects you to play Rorona first.
So with this in mind, starting with
Rorona is a good choice because it most naturally leads into the two best games in the franchise (Totori and Meruru). It's also super accessible, with really simple mechanics and an extremely generous time limit. But one big problem with this is that Totori in particular has way more issues with sexualization/fanservice than any other game in the franchise, so I wouldn't recommend it to people that find that revolting. And even in the other two games, you're going to have to deal with by far the most offensive and disturbing character in the entire franchise.
The
Mana Khemia games are also pretty good as an introduction, especially for people that want to play something that feels more like a traditional JRPG and don't mind playing PS2 games. Because they honestly feel more like a standard JRPG with light crafting mechanics. The also aren't super cutesy like the rest of the franchise, so they're an obvious choice for anyone turned off by that. But this does mean you lose out on some of the defining features of the franchise like more open ended exploration and a slice-of-life focus.
You might also find this
link useful. It gives a brief overview of every game, including recommendations on where to start. Plus it comes with a bunch of other important information, like how to pronounce "Atelier". Which is something even the official dub can't get right despite there already being like 20 of these games.
In general though, I think a lot of people make these games seem way more complex than they really are in an effort to attract fans that would otherwise be put off by their art style. Only Totori has a time limit you can actually fail, and the rest are really easy to get into. If anything, the problem with the games is that they were too easy before they started giving them difficulty options (and sometimes even with the difficulty options). They're supposed to be easy and relaxing casual games after all.
