Random points: If they need to stop in Cardiff a night, River House Backpackers is extremely good value for money - hostel rates, hotel quality. Also there's a greasy spoon next to the bus-stop in Merthyr that is extremely cheap and does these huge kebab burgers etc. The climb after Talybont Resevoir is pretty stiff - I saw people walking up. Conversely, for me heading north, I had to walk down as the brakes on my bike weren't up to it. So there's that to make sure of also.
Once you're at Merthyl it's *mostly* a gentle decent all the way down into Cardiff - mostly signed and not that hard to navigate, however, there's the odd point where it's you can loose the route if you're not paying attention. Passing thru Ponty is one iirc. Also lots of speed gates on the route, which seem to go OTT in places. I don't recall needing a map at all for this bit.