I just find they aren't very aggressive. I'm not in a good position to say everyone should be super aggressive, I'm just trying to say that being so just a little bit could help. I did some 1v1s with them and saw this. We did splat zones using the same weapon and while I was down securing the splat zones, they were still in their base.
We did a pb turf war and I was able to sneak up on them without them knowing, even tho they were looking in my direction. While they were standing there waiting for me to pop out (they were waiting because I hinted I was there), I used an ink strike behind a crate and slatted them. I don't think they are very aware of the surroundings because of this.
Ahhh, kinda idling and not pushing, huh? That isn't good with a heavy weapon either, you are bond to move and act as a support unit with a heavy splatling. Did they even bother doing single player? That should kinda push them to use different weapons, and coax them into how to use tactics around the turf.
They also probably need to try recon, and the try out room, if they haven't. Because not knowing the map is going to encumber them later, and that should be a hint towards them.
I will be honest, I am not a splatling user, but I am a brush, slosh, and roller. With the way they are acting, they will end up being pulverized and end up ragequiting, no offense. (And I know you are trying to chime this in too.) But yeah, get them to play single player, they really need to learn how the game works. The only thing is that splatling doesn't come till much later in it, but at least they can get a work out of a regular shooter, and some other weapons. Because they are going to get BORED.