According to the BBC article at https://www.bbc.co.uk/news/world-us-canada-56149894,
"Boeing has recommended grounding all 777 aircraft with the same type of engine". So this isn't just a few airlines or an individual country.
Boeing doesn't seem to be at fault here. The particular engine is from P&W and they make engines for Airbus and others as well. I can't see how any of this should be extremely negative long term for neither boeing or P&W.
https://en.wikipedia.org/wiki/Air_Mail_scandal
United Technologies is the result of the manufacturing interests east of the Mississippi and Boeing got everything west.
It just sucks for them to be in the news again for airworthiness. I might considering investing in them right now to be honest.
Why would you expect grounding 128 planes, nearly half of which weren't even being used, due to a problem with a part that Boeing doesn't even produce, to affect Boeing stock?