I do not really understand what the FAA means by “inbound international flights will be managed on a case by case basis.”

I am watching planes stacked up on final approach to land at KBOS as I type. From flight radar 24 they are a mix of foreign and domestic. So incoming flights are clearly being managed here.

Does this imply some potential risk from inbound international air traffic? Or maybe the FAA meant outbound? Something else I am missing?

I suspect they mean that inbound flights from international airports may still take off (probably in part because the FAA can't _really_ stop them, in part because the volumes are much lower and thus more manageable, and in part because one might hope that by the time it takes to get to the US from many international destination, the incident will be resolved).

An Eithad flight took off from KBOS after the ground stop

Flight EY8 from Boston to Abu Dhabi https://fr24.com/ETD6PX/429055e2

It likely means any inbound international flights can be told to turn around and/or land outside of the U.S. instead.

The intent around the ground stop is to reduce in-air U.S. traffic.

friend is flying from London to NYC and they were told they might need to land in Toronto.

So my guess this is what it means "case by case". Some which are close to destination would be manually routed, others will land in different places.

Maybe they mean inbound international flights may be allowed to take off from their origin, on a case by case basis. Usually a clearance is required to take off from both the origin and destination.

Presumably it means they may allow some international flights to the US to depart if they expect the issue to be resolved by the time they are in US airspace.

Some might be turned back, depending on how far they are in the journey, fuel, etc:

like you don't have enough fuel to fly back across the ocean they'll accommodate you I assume?

I think it's more mundane than that, I assume they mean that certain international flights will be turned around or redirected while others will complete their route. Say a flight just an hour underway London to New York might be turned around while one nearly at its destination may be allowed to land.

Depending on the nature of the disruption it may be they simply have drastically reduced capacity to handle traffic so landings may be much slower and they'll manage the traffic based on various priorities.

Probably just means that if you're less than halfway across the Atlantic, you can turn around and go back to Europe. I really wouldn't suggest reading too much into the phrasing.