TS3 dropping a client


I am renting a server from a provider and been doing it for years now. All is good, except a friend of mine has connection issues a LOT of times.
He gets the “ping timeout 19 0 0” message and won’t reconnect until manually doing so. I’ve been trying to read up on this and every time we tried something, it didn’t work. Today he tells me about TS5 and I found promo videos and discussions about it, but apparently he has some version of it (I’m sorry for mentioning this, I know nothing about how this beta works and I personally don’t have it, so I don’t know whether it’s pirated or open to the public), and he had ts5 and his now reinstalled ts3 client running side by side and another computer on the same network off the same router running another ts3 and miraculously only his main PC’s ts3 client dropped connection with the usual error out of the 3 instances.
Does anyone have any ideas why this might be and maybe what to do to change this? Until now we thought that it is some internet or maybe windows setting, but now it looks like it is indeed in connection with his TS3 application.
Thanks for reading.