Thank you to all who responded, much appreciated.
Honestly, for a lower-salary young person I ignore cash-back cards and simply get a standard zero-fee credit card from a major company that (hopefully) won't be bought up or bankrupt in 3-5 years.
The reasoning is simple; with a moderate salary and short credit history the amount one can reap from cash-back cards is minimal without churning. Churning is more of an 'advanced' skill, and benefits from having both a longer credit history and more available credit (neither of which this person apparently has). I mention "from a major company" because I struggled early on as several of my oldest cards were closed when the issuing bank closed. At one point i had 15+ years of continuous credit use but my report said my 'average credit age' was < 2 years because of this.
Thanks for the response, you make a very good point. That makes another plus for Capital One, Chase, and Citi.
I like the Citi Double Cash, which doesn't have a signup bonus but offers 2% cash back on every purchase.
They could also try applying for one of the Chase Freedom cards (I'd probably just do the Freedom Unlimited for simplicity's sake), which would give them a good start to Ultimate Rewards points if they think they'll eventually try for bigger, travel-focused cards like the Sapphire line.
I wasn't aware of the Citi Double Cash, thanks for the tip. I've used Chase Freedom myself, but I'm not sure this young person plans a lot of travel just yet.
Chase Freedom and Chase Freedom Unlimited have good bonuses on their rotating categories (which usually include groceries for one quarter) and earn you rewards points that you can save and eventually combine with the more splashy bonus cards if/when you want to apply for them.
If they are outdoorsy, the REI branded card from US bank gives extra REI dividends and also provides cellphone insurance if you bill your phone payments to the card.
Fidelity has a 2% cash back card that I regularly get offers for.
I was also not aware of the REI-branded card, thank you.
I use Fidelity myself for my HSA and Roth IRA, they also regularly offer the same card. I find myself wanting more and more to move my business to Fidelity, particularly for their brokerage services: commission-free purchases of stocks & bonds, the fractional share purchase option (especially great for young people starting their investing, whether investing in index ETFs or stockpicking), free dividend reinvestment, and the no-bs fee-free maintenance of my HSA. Interactive Brokers does offer a better margin interest rate, so I'll continue to use IB as well.