It is both friendly for solo and yet not at the same time. I will TRY to explain
GW2 overall the players are super nice, and helpful, but there are things you NEED other players for like big Meta events that cannot be solod. But they can be played solo in terms of you don't need to be in a group. But other players must be there to help.
Raids are likely the only piece of content you truly cannot do solo, very VERY few Pug raids do well.
Im gonna ramble I can tell already
PvE: Can do 99% of the game without a group or guild, only Big Metas, Dungeons, and raids cannot be done alone
PvP: Can be played solo, but is more fun in a premade imo
WvW: I can't say I haven't done WvW in years, but back in 2013 you could solo roam and pick off other roamers. Or join in a big Zerk event
Gw2 is a very different MMO. The traditional goals of getting new and better gear isn't quite the same. Dungeons can't be solod as you get scaled to the level of the dungeon, so even at lvl 80 in max gear if I go into the first lvl 30 dungon, I am now a lvl 30(with better stats than I did at 30 due to my gear but still a lvl 30)
I am in a guild for new and returning players, but I mainly play with them on Monday night for missions(like guild group quests), and fractals(like high skill mini dungeons). But other than the GUILD missions, I don't need to be in one. I have solo qued for fractals without issue.
Ok rambling over.
TLDR: Very solo friendly, I play alone for the most part and do not feel I am missing anything