Prestashop?? Unique predicament

A company doing $50 mil should be able to hire a professional consultant. The free advice you'll get here won't even come close. Try hiring Ron Rule.

You don't really provide much information. It may very well be that PrestaShop is the perfect platform for this business.

The first step is usually to define what the business goals are and then select the various tools and platforms needed to achieve those goals.

Regardless, the programmer should be able to articulate which platforms were considered and why he selected PrestaShop.

I ABSOLUTELY agree that a project of this type should be run by someone experienced in ecommerce. Its almost inevitable that a rookie will make avoidable (and possibly expensive) mistakes.

Stripe + Prestashop could be enough to do the job, but is the lonesome and inexperienced developer enough?

Something about this situation doesn't really add up, though. Are you sure you have the right amount of zeroes in the yearly sales figures?

Depends on number of products. I am business of both Magento and prestashop here are the sites
Free & Premium Magento Extensions, Modules & Services by FME For Magento
PrestaShop Templates and Themes - Prestashop Modules and Addons For Prestashop
Speaking from experience I would suggest you to go for Magento instead of Prestashop because its ability to handle large number of products.

Use stable Prestashop modules and professional Prestashop service that we can offer

The fact you have a in house programmer is not necessarily a bad thing. If he is a PHP programmer, most likely they will choose Prestashop, Opencart or Magento. And if he is a programmer, and not necessarily experienced with Zend framework, it is probably why he is avoiding Magento.

You have not told us what platform you are on right now that you want to make the change over from. Is it an old platform like zencart that needs to be updated? It could be why he chose prestashop since both are PHP and are similar in many ways.

If he has a mandate to keep costs low, it might be why he chose Stripe. But if you are doing 50MM a year in business (retail?) you might be better off springing for the gateway since at those volumes you can really save a lot.

That being said, you do need a business person to drive the project, figure out the needs for the store and have your programmer go from there. Don't necessarily agree with hiring a consultant, no one knows the business better than the business people, only thing a consultant can do (IMO) is help you figure out your menu structure, help with SEO and descriptions.