The main no no's:
No helmet.
Standing up in fast moving water.
Paddling way to close to the lead boat.
Not knowing how to "read" water or they would have known avoid that big pour over that flipped them both.
I'm all for getting out there and having fun, even doing dangerous stuff that can kill you, but you should minimize the risks as much as possible by equipping yourself with some basic knowledge and skill before you jump on a fast moving river with slow responding boats. The standing up in the river really made me cringe. I had my foot caught between two rocks on the Middle Fork of the American River in 1985, I was held under for 15 20 seconds according to the other guide i was with but it felt like and eternity. I pulled like hell to get my foot out and lost my shoe as well as some skin but it was a wake up call. I didn't even try to stand up when i fell out of the raft it just happened during the tumult of the swim. A few years later a expert kayaker drowned after he took a swim form his kayak on the North Fork of the American River and his foot became wedged between two rocks. This type of death on rivers happens much more than you would think.
The river they did looks like a fun river with some great surf waves and play spots and I would hope they get out and do it again soon, but next time a little better prepared. We all make mistakes when learning new things, the key is not to repeat them.