OK< just to be clear,
the ONLY way to get a sunk motor RIGHT, is to drain oil, and SPLIT the cases and CLEAN the inside,
MY bet is he DIDN"T do this, when that water got in, it also brought in some crap, dirt sand or???
and after he DRAINED oil, or FLUSHED motor with what ever he used
it ALL didn't come out,
he re filled new oil, ran motor and that crap inside, worked its way into places it shouldn;t have been,
and now he has motor issue's
I'd start with a compression test, and see what its at now?
the name of the game when you sink a motor, is only ONE RIGHT way to resolve the issue's and again, that's splitting the cases and cleaning 100%,
anything Other than doing this, is a GAMBLE< it may work or not NOT.
its just how it is, NO short cuts that are fool proof?
SO< did he really do everything needed after sinking it??
I;m again, guessing NO he didn;t split the cases., so??
hence, where he's at now?
sorry NOT what you want to hear?
BUt possible issue's could be bend valve 's, or a top end on its way out!