Almost everyone who tries to get non-gamers into gaming makes the exact same mistakes with the exact same games. Portal isn't a beginner's game, nor are most 2D platformers.
------
Others have already commented on specific things the girl did or failed to do so I won't get back to it, but come on, we are dumbfounded that she doesn't seem to be picking up our decades of experience over a ten-day YouTube experiment. Just how good were we after years of NES platformers? I bet most of us have white whales we never managed to beat, instead spending months fucking around the first levels because we were too clueless to make it further.
However, there is an elephant in the room: you cannot just handwave away that she is a girl.
Grimlorn is 100% correct, a guy would look around, experiment, try more things, be more daring, random even, not to mention our God-tier ability to interiorize an abstract representation of a partially known explorable space. Mere navigation is a nightmare to many women, gaming is a male thing that appeals to male skills; when was the last time you saw a "gamer girl" who got there on her own violition and not because she got brought in by a boyfriend? The girl in the video agreed to sit down for something she doesn't give a shit about, and for a "non-gamer" she is not doing exceedingly bad.
We underestimate how much everything looks like white noise when you are brand new. Yes, you can single out the yellow line or the quest compass at a specific point in time when you are reminded, but after that your focus get split everywhere else on the screen and you might completely forget about it for a while. It's the honeymoon phase, you're curious, everything is new. The fact that the extreme graphical density of modern games is overwhelming says nothing about her, or the way modern AAA games are designed or supposedly fail to convey their intentions to the player. She's just new.
------
Since girls don't care about gaming except as a bonding activity, that means the foot in the door should be cooperation anyway.
Cooperative gameplay is very accomodative of boyfriend/girlfriend dynamics. When in doubt she can always just follow you, when things get tough you can protect her, you can clean up after her mistakes, you can always teach her, correct bad habits and misconceptions before they take root, etc.
I insist that videogames with your girl is a bonding experience first and foremost. No one gives a shit how well you think you should be doing, don't get mad at her for underperforming (and by God she fucking will). The point is that you are playing
together. Girls aren't interested in a challenge, in number-crunching or in the thrill of victory after repeated failures. When you play with her, you are in casual mode, you are always underperforming. No one fucking cares, you are playing together, I'm sure you play on your own when you're not together, odds are she never does, so when you are together it's her show.
Make it interesting to her, if there is something fun like an overpowered plot-mandated weapon or a vehicle and you think she won't just waste it, let her have it. If there's a cool first-person animation or cutscene, let her trigger it. Be the Call of Duty sidekick who's always slightly ahead of you, standing next to the door, nudging you towards the next objective, hinting at what needs to be done. Soften up tough enemies so she gets the killing shot, verbally communicate your state of mind ("wow, that one looks tough" means let's be careful), block trashmobs so she can tell shields are useful, dramatically sidestep to cover in order to reload safely so she can read what you are doing and learn by mimicry, and by everything that is holy, never, ever get mad. No one learns cleanly under stress or mockery, and most definitely not a woman, and most definitely not a woman trying something she doesn't care about in an effort to be nice.
------
How do we learn to drive? First we (informally) observe drivers our entire childhood. We have a vague idea of what cars do and how a car behaves when it's driven properly. Then we get some verbal training, then driving with heavy assistance, then less and less assistance until autonomy.
Videogames aren't voodoo magic. Adults can learn to operate 1-ton missiles made of steel and plastic moving at 90km/h zooming past each other separated by a dotted line. I'm sure they can pick up videogames, it just takes time and giving half a shit.