I have almost all my server mods in both server and client because it's easy to track updates and all that.

This mod says it's client-side, so I said to a friend with a low spec PC he could disable it, but he says it's required to connect to the server.

I can delete it from server, but first I wanted to report the issue :P

Yeah I guess this is a side effect of how custom sounds work.
Since this mod has an option to use a custom lava drip sound, if you put it on the server it registers the sound which makes it required on the client.
Right now in order for the old custom lava drip sound to work, it needs to be on the server in multiplayer because of the way Forge does its registries (#1). I need to figure out how other Forge mods do custom client-side sounds and then I can make it not register the sound or anything on the server.
I plan on doing a rewrite to combine the Forge/Fabric versions and make updating easier, so I'll probably fix it when I do that.

But until then the work around is either removing it from the server, or have your friend keep it installed and just disable it in the config menu (should be almost no performance impact).