If it's only a specific player that has the issue it's more likely to be an issue on their end, not the servers. This could be a lot of things though - poor game performance (not enough RAM allocated to it, high video settings, faulty mods being installed etc..), poor PC or internet performance at the time, outdated java or graphics drivers etc...
Allocating more RAM to their launcher, lowering their video settings, making sure Java and their graphics drivers are up to date and closing anything else running on their PC at the time would be advised to see if it helps.