My friend started up a server the other day so we can play on it.
After a few hours of playing we wanted to build a slime farm, he typed in /seed and it wouldn't let him view the seed

Can anyone help fix this?

I'm not sure about the "seed" part, but if you want a slime farm, you can build in ANY biome(On PC version anyhow. I think Minecraft on mobile and consoles have limitations) and have slimes spawn.
Just needs to be a large flat area under 40y with a roof equal to/higher than 3 blocks.

I thought slimed only spawned in certain chunks? Or did they take that part out? (We just got back into playing)

Google Admist, it can help.

If you're on pc, they'll spawn just about anywhere in large flat areas under 40y where the roof is 3<=or higher (It gets annoying since I have gardens underground in my base and they spawn even in them).
If you're on mobile or console, I'm not sure how they work, but it should be pretty close.
Though, I believe if you're in a mushroom island, it may not work as no hostile mobs can spawn naturally there. Only spawners/villager(If you drag zombie villagers there and convert them)/golems can.