Is he only being offered internships in London, or actual jobs with a wage?
If internships, I agree, this is usually a route for those with free living/parental support.
If it's a low paid foot in the door, he should definitely think about taking it -surely that's cheaper than £33,000 for a year, even if parents subsidised him a bit. He can get a house share and live off baked beans, it's not undoable.
I wouldn't encourage him to go for a 1 + 3 which is a funded Masters plus a PhD unless he actually wants to become an academic in that area, or is very very sure he will like the subject. Quant PhDs tend to do very well in the City, but it depends how good he is at maths. If he wants to go this route he would need excellent UG quals (like a first) plus apply all over the country (jobs.ac.uk is the place to look as someone else said).
It's a little upsetting for students to come out of UG uni and find the playing field still isn't level, and those with money/properties/live for free, can do internships all are advantaged- he'll have to box clever in the current climate, but I don't think doing that Masters at Warwick is the be all and end all, he should look for a finance Masters at around £10 grand if such a thing can be had, and finance it through the student loan system. Or has he looked in Europe, although the Masters are still quite expensive, there may be options there which are still cheaper than Warwick/living costs cheaper.
Or, based in the SW you have Plymouth, Bristol, Bath, Exeter plus Cardiff and the Welsh unis for a commutable Masters if you wouldn't mind him living with you (you may!) Many Masters courses are structured so students don't have to come in every day but do intensive blocks of teaching.