in metal in the engine, particularly aluminum as the engine is mostly aluminum.
Certainly removing a tight fitting bearing and pressing another back into the engine can result in some aluminum flakes. (The flakes become very thin and tend to break into smaller pieces after going through the gears of the oil pump.)
Also, if the IMSB install was, pardon the expression, a butcher job, this can result in a real increase in metal in the oil as the butcher job generates even more metal particles as the bearing is pressed home incorrectly.
If the engine was pulled and worked on and then reinstalled and run this can also work to explain the flakes in the oil. The movement the engine gets being removed and worked on and reinstalled stirs up debris on the bottom of the oil sump and when the engine is subsequently run some of this debris gets ingested by the oil pump and what you get is a jump in metal pieces in the oil filter housing oil and oil filter element.
As long as a magnet run through the oil doesn't come out fuzzy with ferrous particles and the engine is otherwise quiet the owner can change the oil and filter and run the engine until warmed up, just idle is good enough, then remove the oil filter and see what's present.
What he finds or doesn't find determines what he does next.
I have to add that he should be ready to switch off the engine ASAP should the engine suddenly start making scary noises. While I don't think this will happen he should be on the alert, just in case.