My friend can't install mods or join modded worlds

[JosePapp]

Hi, in playing with a group of people, and one of my friends who hadn't played in a long while CANNOT install any mods at all. If he ever tries to join a world it says that the world has outdated mods. 

We've already tried reinstalling the game, unsubscribing and erasing the mod files AND the workshop files, allowing the game through the firewalls... Everything, but maybe restarting his computer before doing that (doubt that would help) same with the 32bit version.

Apparently  whenever a client mod is installed it copies over to the server mods or something, but it does it all corrupted like "workshop (numbers)" and they can't be uninstalled or updated