The problem is, the server contains ~50k bricks. How much ping do you think it could get? (estimation please)
All Blockland servers will have the same ping, no matter how many players or bricks are on them. That's how the internet works.
From my location in New Jersey, I average about 75 ms. It does not scale linearly, though. For example, if you live in California, you'll probably get about 115 ms.
I think you could hold about 35 players with that many bricks, if it was a DM/TDM. If it was something less CPU-intensive, like a city RPG or a freebuild, I'd expect 53 players.
I really don't think your friend will have to worry.
Premium servers also get a higher CPU priority than free servers.