All Friends But One Can't Join My Forge 1.7.10 Vanilla Server

[DGPlayer]

Hello,

 

I need some help,

 

I made a 1.7.10 Forge Vanilla server to play with three of my friends, two of them joined without a problem, but one of them joins and loses connection immediately.

Can his internet connection be slow and that's why he can't join or is there other possible reasons?

 

Max players is set to 20

I run the server with this code without a problem: java -Xms512M -Xmx7G -jar minecraft_server.1.7.10.jar pause
He isn't banned.