5. Technical Support calls.
6. If the server hasn't failed / crashed / died yet, it will soon.
7. If the datacenter hasn't had a power outage recently, it will soon.
8. If 6 and 7 don't happen, your users will delete some critical files and take themselves offline.
But seriously, even though web hosting is one of the most competitive online markets and you really need to know what you are doing for SEO / SEM to compete, there are pretty good margins and lots of easy add-on services to make money on these days.
Plus now all the cloud hosting companies have APIs, so if your friend is technical enough he can build his own front-end and make it look like he has his own hosting company -- no infrastructure headaches required.
--David