Generally this is caused by stability issues in his connection. He can try using another FTP client, or uploading through the web after all.
The main reasons we do not recommend uploading large files from the web are:
1) There is no possibility to resume if the connection gets interrupted
2) Uploads are not implemented very cleanly in browsers. That means that for a multi-GB upload, your browser will also start to use a couple GB of memory, which can cause a PC to slow down or even hang. In this day and age 850MB should not be too much of an issue though.
