I recently tried oxid eShop, which I really liked.
It is quite simple, but very flexible and extensible.
It is very easy to understand and comes with a nice AJAX interface.
Also, its design can be changed using the interface editor, which does not require you to know anything about programming.
If you want to look at it first without installing it here, a link to the store and to the admin area . The login for the administration area is admin / admin (user / pass).
It may not be like a full-featured / professional one, for example Magento , which is a full-featured store system, but I'm pretty sure that exactly what your friend needs.
It was also featured in the July php release | architect .
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