Friend unable to join custom modpack

Wanna send us the log even there isn't any special things?

Also this could help:

https://forums.minecraftforge.…connection-is-terminated/

I found this:
*Quoted*

Computers, (PC, Laptop, Mac, etc) all have a finite amount of memory. Think of it like blank paper to do a math problem on. Everything you do is taking up space on the paper. You can turn off programs (erase parts of the paper, to keep up the metaphor), but there are always limitations.

If you host a server on your computer, that's taking up a sizable section of memory (paper). If you open a server and a client, you are effectively having 2x the space taken. If your friend connects, even more is being done. As Grimm said, you're overloading 6 seconds after loading the server, so that won't happen.

The same thing goes for modding. If you make a "modded" instance, you are increasing the amount of memory (paper) that is taken up. If you're computer can't think because its out of space, it will stop calculating everything and focus on essential processes, which minecraft is further down the list. This can cause crashes, especially because Java is known to have frequent leaks (think of it like losing parts of the paper to an ink spill) that require restarts. If you're crashing, you may not be able to handle the mods you have in it.

I dont know if this is helpful!

To play a modpack on a server, you need all of the server-side mods. So, since your friend was missing a server-side mod, he was getting terminated.