My guess would be that your cousin's ISP is running some sort of transparent proxy that's doing virus scanning. Whether it's finding a virus and breaking the connection or it's broken, or it's something else causing the problem I don't know.
One thing I can think of that you can try is looking into if there are any encryption options you can turn on. If the data stream is encrypted, the scanner can't do anything. It doesn't prevent it from trying, though.
Another idea would be to try using a different port. It's unlikely that their transparent proxy is looking at anything but the default FTP port. (Yes, I know FTP uses multiple ports, but if it's not looking at the control channel, then it can't magically find the data channel.)
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Bitt Faulk