I think there are probably differing views on this but in terms of career prospects not significant enough difference to override all other factors so that he can really make a 'bad' decision here.
Does the Warwick course have particularly strong links for summer placements? That would certainly be a tangible benefit.
He needs to be happy with the lifestyle - very different at the two places. If he chose Warwick originally because he prefers it as a University to Durham that's another factor. If he prefers the maths/econ as a course and would be happy with the traditional collegiate style at Durham then go for that.
Does it make a significant difference as to how far he is from home? Would that bother him?
Remember, it's a happy choice not a stressful one