It's not a glitch, game-altering commands are disabled in the multiplayer (spawning items/mobs, changing weather, day/night commands, etc..).

The only way to use them in the multiplayer is by using the SkToolBox mod. You can download it from Nexus and install it manually or get it through the Thunderstore Mod Manager.
https://www.nexusmods.com/valheim/mods/8
https://valheim.thunderstore.io/package/Skrip/SkToolbox/

Keep in mind this is client-side only, so your friend will have to download the same mod if he wants to use console commands as well.