Yes, with some guidance and thoughtfulness
My experience is a bit secondhand, but I'd say yes.
While I've not played Tales From the Loop I am familiar with its mechanics, and I work with 8-12 year old kids in creative workshops regularly, including with games and roleplaying games.
If your friends' son can handle playing boardgames with adults, he can handle RPG rules, especially simpler ones like those in the Tales From the Loop starter set. (Though of note, I'm pretty sure the rules are the same as in the full game.) Be mindful of the content of the game though; while it's fantastical, it's also rooted in the real world. Things that are fun to explore as an adult looking back on childhood might not feel that way to an actual 9-year-old. Also a good idea for whoever will GM to read through the adventure and check in with the parents to make sure its themes, sci-fi and mundane, don't step on anything that they find especially challenging.
Where they may need some more assistance, at least to get started, is in collaboratively imagining a world and story (especially ones with 1980s constraints which will be a bit foreign to him), understanding the boundaries of what they can do in a more open game, and how to work cooperatively with other players as a team.
While it sounds like he's definitely hit the developmental milestone of embracing rules as constraints which ensure a fair competition, the unwritten rules around roleplaying are not always clear to children and young people. With teenagers this often becomes a problem of inter-party conflict - they are testing boundaries and might stab a friendly player character just because they can, not really thinking about the consequences for the player of that character - but with younger players its usually more the case that their imagination will take them off somewhere that's not really connected to the plot or situation. (Doubly true in Tales From the Loop, which focuses on mystery solving and not usually "defeating the bad guy" or physical conflict - something lots of boardgames do.)
So I'd suggest clearly explaining things like the expectations of a roleplaying game, and making sure they understand the fictional goals when they're established. Don't expect them to necessarily speak in character, or play a character that differs much from themselves (it might be best to give them first pick of the starter set characters), unless they have an established interest in storytelling or acting. The important question is always framed as "what do you do?" to make sure they're on the page of controlling a character, not just making up the story.
You can use techniques during play to help them make decisions that respect their autonomy but help them with suggestions. One I use often in creative workshops when a young person is stuck is to offer them a couple of options which are fairly broad. Often they won't like either of them, but it helps them find the scope of the question and also to realise that they do know what they want to do, they just didn't feel confident in saying so.
I won't go on, because what help your friends' son may or may not need is impossible to know before you get into a game with them, but I would say yes: in general its appropriate for a 9-year-old, so long as you check in about the themes first and are prepared to help guide them through their first roleplaying experience.