Thing is, how do you determine when you are ready for ranked? This guy had around 160 games on Riven, and probably even more in normals. If you only play normals on a champ that you're wanting to master, how high up should you get that mastery?
Normals has a more chaotic environment than ranked. In one game, you could go up against a smurf on their main champ, and the next, you could get someone who is literally playing that champ for the first time. Not to mention the amount of cheese picks and builds there are in normals.
In ranked, you are far more likely to go up against people of your skill level, of whom is playing a champ that they are at least halfway familiar with. I'm not saying that you won't see the same weird picks in ranked as you do normals, but you will see it far less often, even in down in the chaotic trenches of Bronze.
So, in my opinion, ranked is actually the best place to master a champion. Yes, you should play a few normal games, get the basic combos down, and get a feel for the champ, but once you have that, you should certainly head into ranked. I say this because I've done it, specifically with Gnar. I played maybe 10 games total with him in normals, if that, then started to play him in ranked. I now have him at 100k mastery, and overall winrate of somewhere around 55% with well over a hundred ranked games on him.
Now, I understand that Riven is far, Far, FAR more difficult than Gnar ever will be, but if this guy is aiming to master Riven, I'd say he's doing it right. It'll be slow, and it'll be painful, not only for him, but also his teammates (but, more so for him). Give him another 160 games on Riven, and he may very well have long climbed out whatever elo he was in.