You’ll definitely have to have most of the client side mods installed before you’ll be able to connect.
I’m not sure if there’s a code option in prism. I know that if he can give you the actual mod list, you can just manually search for an install the mods. All the modded servers I’ve played on were using a named modpack, so I just would search for that pack and install it.
I have limited experience here as well unfortunately. He shouldnt have to send you the mod files themselves, if he sends you the mod names you can download them directly in prism launcher.
You might be best off checking with @[email protected] since he seems to have more direct experience with this.