My DH is a physio. Ice bath. Buy bags of ice! Add cold water. Lie as long as He can bear! Up to 20 mins. Repeat next day. Although he will want heat! Ice is the way to go. Expect pain to be at it's worst 1 1/2 to 2 days later. Keep moving, especially tonight, keep waking and gently stretching
Sorry he is in agony. Must admit I was a bit worried when I read the thread title - do you really mean he did no training? Or didn't train to that distance? It is (apparently) v v dangerous to do that sort of distance on no training.
He could run 10k reasonably comfortably but hadn't done that for about 6 weeks. I did tell him not to run the whole thing, but he clearly had other ideas. I will encourage make him have a cold bath tonight. Other than that he's eating lots of protien and drinking loads of fluids, including sports drinks.
Why could it have been so dangerous? Is it that he could have badly damaged his knees?