Their relationship is either creepy or harmless depending on how you look at it. Ruby's post seems harmless but if I said, "18 year old Norwegian student travels to UK to meet popular 21 year old YouTuber who befriended her whilst she was still a minor, has been writing letters to her for years and has helped promoted her channel" you'd think differently to "two university students, aged 18 and 21, spend day in London together".
We can't know exactly what their relationship is like but we do know that Ruby was as an adult contacting this girl whilst she was still under 18. We also know that Ruby seems to want more children as penpals, perhaps because the current ones are becoming adults who are less adoring of her but there is always the possibility of something more sinister.
Ruby does not appear to be in very much contact with any of the YouTubers she claims as friends prefering the company of people who adore her or who she can manipulate (including her parents). We can choose so see her as a damaged girl with an eating disorder and possible OCD but we also have to consider that she is manipulating everyone to follow her narrative. Pity play is common in female narcissists and Ruby is certainly trying to get pity (unwell, bullied, homesick) whilst also getting seemingly angry at anyone who is doing better or getting better grades than her. She has a very self-centered world view, most of what she does seems to be to try and get people to think she is a good person (vegan, shopping secondhand, supporting charities) but the truth of the matter is you know she doesn't really care about any of it.
Maybe everything is perfectly fine but if Ruby does something bad at some point it's not like people can say there weren't any warning signs.