How he tells the story depends on what he does, so that’s why he’s struggling: you can’t tell a story that you don’t know the plot or ending to yet 
Right now he should concentrate on finding a job; once he has one, and he’s in a position to apply to graduate school, then he can start working on what his story will be.
But one thing I would advise is for him to shift his thinking: this isn’t a “gap year.” This is just his life.
LOTS of people work before they pursue a graduate degree for a variety of reasons, including that they just don’t feel like going to graduate school right away. He doesn’t need to explain why he didn’t go straight from undergrad to grad school, as if that is some kind of bad thing.
If he’s able to find a research job in his field, that’s great - he can easily write a story about wanting to pursue a research position to really immerse himself and how he fell even more deeply in love with the field or whatnot on that. If he works a job in an unrelated field, he doesn’t even necessarily have to address it. Especially if he only takes one year - he’d only have been working the job max a few months before applying anyway.