Note the CSS is a realtime blacklist, while most mail providers would not use this on a submission server, the fact that your friends IP address is on their indicates that a device on their network is sending spam straight to mail exchangers. automatic delisting occurs 12 - 24 hours after the last spam was seen.
So the frst thing to do is to check ALL devices on this network for malware using anti virus programs and also something such as malwarebytes to check for other types of malware.
You can try delisting the IP before then, but unless you get to the cause of the problem, they'll end up back on the list.
As a Very Insightful Person, I'm here to share my knowledge. I don't work for Virgin Media.