Getting Disconnected when Entering Instances

Although it doesn't happens 100% of the time, my friend keeps getting disconnected with no error, just server disconnection, and it happens very often, I'd say 75% of the time. And it's not only dungeons, but any other content where you are out of the "free world" like houses, dungeons, or places like The Rising Stones or the Reisen Temple. It just disconnects them to the main menu with no numbered error like "2002" or "4004".

They tried moving the game from the hard drive to SSD and even fully reinstalling the game. We know it has something to their connection to the server, but we are not sure on how to fix it. If any one has any idea on how to fix this, it would be greatly appreciated.