When does the "main story bus" stop rolling?

[Omedon]

This is going to be more brief than I prefer, but: 1000 characters.

Simply put, does there come a point where the story quest lets you go do something else for a full level or two, or is it pretty much an unbroken (within a single level) chain to 50? I have a friend that is "afraid" of doing anything else with friends for fear of getting "behind" in her story quest. She'll never finish it (she wouldn't want to waste time on something non-logistically relevant) if she discovers she's 3 levels behind it at some point, but would like to be able to play with her friends in between story quest catch-up sessions. Until Buscarron, she never saw the "red meteor of go-level-up," and she was so happy when she did.

So, does it "slow down," and if so, when?

Thank you in advance for all non-patronizing responses.

Edit: Bonus question! After the grand company, are there any more "features" unlocked specifically by the story? Dungeons don't count, we're not hinged on having ALL the dungeons unlocked. We're also not endgame raid-progression minded, so hardmode primals aren't really "a feature" to us. I'm asking because she's contemplating just ditching the story quest forever.