Raw water in cylinders - Whitby 42 - 1974

Please excuse my ignorance. I'm trying to help my brother who is in remote Fiji with a new engine problem. he has a mid-1970s Whitby 42 with a 4-cylinder Lehman and Vernalift muffler.

When the engine is shut down raw water flows back through the wet exhaust and into the aft 2 cylinders and out the intake air filter. He has had to empty the water (and change the oil) twice. If the sea cock is closed it is fine. And the engine runs fine. There does not seem to be anti siphon system - but it has worked with the current configuration at lease since he bought it over 3 years and 1000 hours ago.

Would anyone be able to give us advice on how to address this?

Thank you very much!

The top of the elbow should have a anti siphon valve on the raw water side ,if so , could be stopped up, allowing the exhaust water to be sucked back past the valve while cooling down , therefore water getting in the back two cylinders.
I will be in Fiji on 10/22/19 on a cruise ship. Contact me thru this site if needed