Diesel started with exhaust valve closed

[fatherchronica]

My dock neighbor started his diesel today without opening the exhaust valve. I did not hear it because I am old, but he said it made a backfiring sound and then quit. He opened the cockpit access and found the exhaust riser popped off and some water dripping from some loosened hose clamped connections. I helped him reattach everything and then he tried to start it and it would not start. By then it was dark so I left. What damage could he have done. It is a newish Yanmar that read something in the high 600 hours.