I did a PGDip at City, am Oxbridge graduate, and am, actually, a sports journalist.. but of a very unique type so certainly not broadsheet cross-sport reporter.
I would say if he;s got his heart set on nationals then strongly consider City, it is very well connected that way. Also because he would be based in London he will be more likely to pick up nationals work experience there - when I was there they were VERY hot on work experience, with placements every holiday, to the extent that you really wanted to try and get Christmas placements sewn up before you even arrived there in September.
Also lots of us did night shifts on Guardian online, YouGov etc and am sure something similar would present itself nowadays.
I wouldn't worry too much about the Oxbridge connection, or lack thereof. At City there were a lot of us there, but actually they did a lot to beat any kind of superiority complex out of us. Many of the best candidates in my peer group were RG instead.
But I would suggest he gets involved in whatever student journalism is going on in his current uni, as THAT is where a lot of networking begins. There are awards for the best reporters in lots of sections, and student sports journalism is very competitive, so I'd strongly recommend he gets involved there.
Also local papers are a good way of building up more work experience time without heavy costs involved in living away from home.
I don't know what the City/Cardiff MA includes these days. In my day the PG Dip WAS worth doing because it provided a very useful transition from academic to work-orientated study. We did shorthand & law, which I'd say are pretty much essential, plus govt structure, subbing, and some page layout work. All bar the govt structure (and now the shorthand, which I've forgotten), I still use on a daily basis 10 years on. YOu can learn on the job, but I think you're more likely to leap-frog onto a broadsheet or large regional without going through your NCTJ exams at a local paper if you have something as well regarding as the City/Cardiff courses under your belt.