I don't really understand this sentiment which seems to be widely shared here. Stripe handles all the PCI stuff. What's wrong with coding up a basic store?
Exactly. PayPal is the same way: if you outsource your payment processing to one of these, then you don't have to worry about the PCI stuff. If you're intent on having a "uniform checkout experience", then it's much more difficult of course, but if you don't mind redirecting customers to PayPal to do the payment, then making your own basic store can save you a ton of money in fees.
OP's friend needs a store quickly, and probably wants a stable, feature-rich, ready to use store. OP most likely does not want to get blind-sided discovering that the plethora of currently existing solutions have upwards of well over $1 million dollars of labor-hours (features, QA, documentation) put into them. Magento might even have over $1 billion in labor-hours. Rolling your own out of simple ignorance regarding what exists is already rather dumb, but the surprise finding out that making anything worth using is a full-time job will be soul crushing, depressing, and may even damage OP's relationship.
Writing an ecommerce CMS because you are educated on what exists and want to innovate is a completely different matter.
Catalog/pricing/inventory management
Category/multiple product display/design
Search
Single product page display/design
Shipping options
Checkout, aside from payment (partial flow, or completely externally hosted?)
Order management (how to get orders out of the system and into a warehouse to be packed)
Promotions and discounts (will you have coupons?)
Customer feedback/product reviews
Contact me/us
And that's just the tip of the iceberg.
Source: I work for an ecommerce consultant (I use/live in Salesforce Commerce Cloud/formerly Demandware)