From my experience, this actually happens a decent amount with servers with a lot of mods, like tekkit and ftb servers, depending on a users connection/latency to a server the delay can cause some desync between the client and the server, which with certain mods occasionally throws a fatal exception in the game and causes the client to crash, sometimes to main menu, and sometimes completely closing the client.
I have this happen to me a lot with connecting to a friends local FTB Monster server, the only real advice I can give is make sure your friends haven't modified any of the configs for any of the mods unless the configs in the server have been changed. Any config that isn't solely client-side, when mismatched with the server, could be a reason for these types of crashes.
I've also had some issues where my firewall/antivirus has seen certain operations in the game and decided to end the process, so your friends may decide to allow the java.exe as an exception to their antivirus/firewall, however note that is risky, as many online websites use Java for parts of the webpage, and allowing Java through the firewall could allow external sites to gather information on your computer without your permission.