It (a) assumes no knowledge coming in, (b) provides a linear progression from no knowledge to a portfolio of web applications, (c) lets you move as fast or slow as you want, (d) lets you skip lessons and topics you're already familiar with, and (e) has phenomenal online support. (EDIT: and it's free)
Once your friend has the fundamentals, I'd also suggest checking out http://flukeout.github.io/ and http://flexboxfroggy.com/ . They're fun little games to introduce lots of css and flexbox features.
However, jQuery is largely considered very cumbersome by more experienced devs, owing to being slow in performance and to being a large file include that slows page load times, and a lot of people try to avoid it for that reason. It's worth studying this link to see how you can do all common jQuery things with normal JS.
There's also "smaller" versions of jQuery, which provide the same/similar API without being huge, like Zepto. Zepto is 9.1kb, jQuery is 95. 95 is a lot.
So yeah, long speech on one little gripe, but I feel like it needed saying because that site is probably good for teaching most other stuff you'd need to know.
I whole heartedly disagree. Experienced devs consider jQuery a tool and know when to use smaller frameworks like vanillaJS or something heavier such as React/Aurelia. Also, in terms of web libraries/frameworks, 100kb is not that big.
Someone suggested bootstrap below, which I disagree with for a beginner but has a strong usecase for a programmer who needs to wrap their backend in something presentable. I would consider learning by using something like milligram or skeleton grid or even something much more opinionated like zurb foundation or bootstrap.
This is because, at least for me, most of the difficulty was around 3 things:
* modularity and structure
* actually positioning things
* terms and concepts
The browser is a processing engine which I think is in c++ or C but the point I am getting at is the big issue with onbboarding new programmers/designers is that they have a massive amount of peripheral knowledge to gain.
So, I personally liked envato http://webdesign.tutsplus.com/courses/30-days-to-learn-html-... which as a complete beginner took me 10 days. A competent programmer could probably finish in a day and get the relevenet bits, then use a grid and be pretty good.
tl;dr you can learn 90% of html/css in ~24-48 hours, that other 10% will take years.
On a side note, is it okay just to know Flexbox, or would a requirement be that you can manage without? Without ready made grid classes as well?