Brackish water ops

PCM manifolds last from 5-7 years with flushing on pure saltwater boats. You will probably be ok on those with them having 3 but once in fresh water they might not last as long if you just replaced them. Luckily those are easy to get. Other parts affected first are the thermostat housings,the upper going first. The water gets under the hose where it clamps on and gets trapped ,no amount of flushing ever gets all the salt out,the cast iron seems to let the salt get into the pore's of the metal as well. If your friend could pull a manifold hose off and take a look that would help. If he can't do that just check around the outsides where the gaskets meet,if rust is showing there it will be worse inside.
Here is an extreme example of what happens. This was a pure salt intake manifold,don't know it's history as to age,if ever flushed etc. But here your looking at a water passage by #1 cylinder,thermostat housing bolts up to the right. The threaded boss you see in the port is where the temp sender locates. You can see around the port what the salt does. I ended up throwing it away,hugh chunks were flaking off inside and because I did not trust it leaking around the gasket surface and either getting into the engine or leaking out of it.   
I don't want to scare you off just want to hopefully help you see what to look for. If it has been cared for you will know just by looking at it.