I don't know if I've shared this since you've joined the site but I've absolutely been there. Before my team disbanded, I had been one of my team's weakest players, and I had to be the one making that long and difficult journey. By that time I had ended up becoming arguably one of their strongest players. Picking up a weapon I found simpler definitely helped but it was far from the only thing that got me to my goal. I have a lot to talk about here.
First of all, I don't know exactly how big of a skill gap there is and it's important to take note of that first and foremost. The ideas you have make me think you've already done this but it needs said in case I'm wrong. Someone being hard stuck in the B-Ranks and being grouped with people who are all S+ is very different from someone who simply doesn't position as well as everyone else and isn't realizing how much damage it does to their game plan. What use does aim training do if your teammate is able to hit their shots just as much as anyone else on the team? Like sure, the practice never hurts, but it wouldn't help as much as directly attacking the problem.
You'd need to watch at least one or two replays to get a gauge for this, but if their mechanics are good and you think there's something with strategy involved that's not quite going well enough, then that's worth discussing. Things like improper special usage, poor positioning, overly passive or aggressive play, and so on. You will need to get very specific as to what it is - even something like "position better" is not enough for them to work with because you need to be able to explain both what makes their positioning worse than everyone else's and what steps they can take to fix it. If they're able to implement that then awesome, either mission accomplished or you can start with another step towards mission accomplished right away. If they find themselves hard stuck then it might be worth taking a break before returning.
I guess to that point I should ask, and this is for something for you to answer for yourself, how likely is it that this player is being effected by nervousness or anger or something else similar while they play with you guys? Those can distract from everything both conscious and unconscious that you need to be doing during a game. Some of these problems can be rooted in something outside of Splatoon and can sometimes be even more personal and more difficult to fix than anything you'd see in this game.
Otherwise, if you think it's a matter of mechanics, then your PB target practice idea is legit pretty cool. Ideally they're able to get practice without getting everyone together, though. The two aim drills I've been telling everyone...
- Try to have them stand in the middle of the group of four training dummies in the lobby. Have them look away from the screen or close their eyes, hold their right stick down to make their character spin around for some indeterminate amount of time. When they look back at the screen again they should try and shoot the closest one to their reticle when they look back. Ideally they shoot them as quickly as they can while still having confident, steady aim and without over or undershooting. Once they get good at that they should try to get comfortable aiming faster and faster and faster.
- Try to have them hold their reticle on one of the moving dummies for as long as they can. Ideally they are standing in a different place each time they do this. They should be using The Shoal's dummies from time to time as well since the one in the back of the room moves at a very different speed from the rest.
The first of these is to practice their flick aim and the second of these is to practice their tracking aim. Every weapon that I can think of needs at least some degree of good flick aim and tracking aim is helpful for people playing weapons with better fire rates. Ideally they're getting at least a few minutes in of aim drills even on days where they aren't able to play Splatoon otherwise but I understand this being a bit much for some schedules.
Lastly, this person needs to be willing to play the game a lot. I'm sure this is a very cliche answer but I cannot stress enough that this was the biggest thing enabling me to improve as much as I needed to catch up with my old teammates. This was what put my mechanics on enough of a level to be comparable to theirs. Although, for as easy as it is to say "just play the game," it's really not so simple. Just playing the game will not net you improvement even if there's a very strong correlation I've seen in low-level between hours on the game and general skill level.
For it to do much for my game sense, I kind of needed to have an open mind and to take in a lot of info about the game from everywhere else that I could. If I were left completely on my own without any outside help then I probably would have taught myself some misinformation and learned myself a new bad habit, but Splatoon is a team game with a bigger community than people like to admit. I paid very close attention to and learned from everything my teammates said, I watched a bunch of informative YouTube videos that I felt had some chance of applying to me, and I talked about the game a lot with players who were way better than me. Every single time I went into solo queue it was to try and implement something I wanted to change with my gameplay. Maybe it would take a few sessions, maybe it would take even longer. It's not always going to be easy or quick but that's normal.
At the end of the day, the question this is all trying to answer is "how do i (help this person) improve?" Kind of an age-old question in every competitive game. There's no simple and easy way to get there but I hope this at least helped give you some guidance on where to start. It still will be on you, your teammates, and your coach to further narrow down where the problem or problems lie, as well as on this one teammate of yours to put in the time and focused effort that they need to.
