The main thing you're going to get from Apple is integrated hardware/software support. You might get something similar if you buy a Linux server from a vendor that supports Linux. I've not used an Apple server in well over a decade, so my assumption about their support might be off base.
The main things you're going to get from Linux are compatibility (most open-source applications are geared towards Linux, and any problems you run into will likely be compounded by running a different OS — and, yes, this is a gross generalization), and lower cost.
I think the biggest factor would be what his current shared server is running. If he replaces it with the same OS, he's likely to have fewer migration headaches.