I think you're going to need to find out exactly what options your friend is looking for, and then see if the wp commerce platforms can provide those options.
My only real commerce experience is with woocommerce, and they do have extensions for just about anything you could think of - but you need to pay for each one. Out of the box it supports paypal and other payment gateways are paid extensions. There are also extensions for things like flat rate shipping and all sorts of stuff.
They have just changed their own website to use woocommerce to sell their themes etc, so they must have confidence in it's ability to scale.
I am going to look at marketpress, purely because of the way it integrates with the other multisite options but i don't know if it has evolved to a state where it can compete with woocommerce in terms of functionality...?