Hi,
Welcome to the forum.
Unfortunately we know less than you about your boyfriend's set up - he's also going to read this post and discover you've pulled the cable out.
On the plus side, pulling out a cable isn't a biggie - leave the system switched off and he'll be able to reconnect it once he's home.
If you want to try and replace it, look at the rear of his system and try to find out where cables of a similar colour and type are plugged in. If it's got a red or white connector, these are connected in pairs (one red, one white). Has the cable any manufacturer marking that you can match with another still connected?
Cheers,
Cofnchtr.