He will only get what someone is prepared to offer. Second hand PCs - even relatively new ones - command very little value. If you do a search in the PCA Forums this question has been asked many times before with the answer being similar to mine.
Whether he would do better by selling it in parts (monitor, HDDs, drives, RAM, etc.) is also questionable. In my opinion he would be better to think in terms far less than the £400 you mention regardless of what he paid.