The station is right next to the stadium....Its the big stadium thing not far away..(it's impressive) but as MIL is blue badge, why not drive and take advantage. Cardiff is fab for getting to...M4 to Cardiff Gate (whichever junction that is) (about 45 mins from Bristol assuming you can get out of Bristol, £5 quid rip off bridge tax to the Frogs) Drive into centre, unload MIL etc, display blue badge, all go and watch the game. Easy.
Don't know if it's changed (few years since I lived near Cardiff) but they used to close off a whole load of streets around the stadium on match days - cones, police, etc, so not sure if even Blue badge are allowed to park nearby. Not sure though - I would have thought they might have a disabled parking area somewhere.
If you get the train bear in mind they have a stupid queuing system after matches where you have to queue up to actually get into the station - many a time (eg last FA cup final day) I missed the last train up the valley due to not being able to get in to the bl**dy station!!!
Some blue badge holders are exempt from the 5 quid welcome to wales tax on the bridge - something else to consider!!
I went there last weekend (for Fiji vs Canada....not quite such a big game) and travelled on National Express coach. It cost £7.50 return and was at nice convenient hours so perhaps give that a shot?? You should be able to book online and get an e-ticket you can just print out. The stadium is really close (like a couple of hundred yards) from the coach station in Cardiff, and leaves from the central coach station in Bristol (Marlborough Street)