My friend can't go out of cave

Hi, first i want to apology for my bad english.

So we have a problem concerning the caves with my friends I can go in and out, but my friend can go in but not come back into the overworld. 

this message appears : Server Not Responding and when she want relog into the server she's still in the cave, we have tried with mods and without the problem is the same

And we tried on 2 different computer mac and pc but every time with the same adressIP

So thank you for reading I hope there is a solution this is a large part of the game that is no longer available