We are currently using an old version of vBulletin to power the forum.
We are planning to change platforms in the coming year and move to xenforos software BUT you need more than forum software - you also need hosting and regular updates and some tech chops to keep it all running. the current version of vBulletin offers some of those services rolled into the license agreement. Do some research...
I would think he should consider the underlying OS and move from there. Windows and Linux are the most popular base. There's others out there but that's going to take some some time. Red Hat just started charging for it's software.
He also has to think about hits. How many front ends, how many application and database servers. It may have to be a 3 tier architecture depending on the amount of hits he thinks it'll generate. Usually splitting the load between three front ends, 2 app server and a database server get you started. You can scale horizontally and/or vertically. It's a good platform to get started with. You just have to decide from there
There's also code promotion. The number one thing you can do to discourage people from coming back to your site is a break. That requires testing and reiteration until the breaks are fixed. It should happen in different environments. Developers work in Dev, Testers work in test and users run in production. SCRUM or Waterfall, it should happen. Otherwise he may he may sacrifice reputation risk as well as lost revenue from bad user experiences.
Last I heard the MSFT cloud only charged you for servers in use, so if you overload the other servers kick in. When traffic is low, they fade away and you won't be charged for their use and they'll possible use them for some one else (multi-hosting). AWS is another option and one I know little about. The downside: if your sharing the server with someone else, you're performance could degrade and that's not good for business - no matter the provider.
I ran a 3 business-class server farm in my den. It ran 3 virtual farms: dev, test and prod. They had 3 front ends, 2 app servers and a database server. The DB server ran 3 partitions for each of the environments. It never got enough traffic to capture performance data but it served my purpose.
There's a lot to think about. Either jump in with both feet and take the plunge or do your homework - just try not to get caught up in analyses paralysis.
Look into Dell refurbished servers. Not a lot of investment to see how it all goes. They'll run windows and linux (unix), so no worries there. The I/O core may have be evaluated before you pull the trigger. You may need different drivers if you go unix.
Good luck.