If you are copying the signal with a low balance, it could be a margin stop out.
Give some more details on balance, leverage and order's lot size.
If you don't give some more details, we can't guess.
Trades are not closed without a reason, if you are properly synchronized and you have a valid signal subscription.
So why he didn't contacted you first and he is asking here?
You seem to know the reason for this problem.
It would be interesting though to check his VPS for failures or restarts too.
Have you seen his VPS log file?
Maybe the same happened on his end too.
We can't say if we don't see a VPS log journal.
First you have to find who or what closed the trades and for that you have to examine the log entries for the closed trades.