[Fledermaus]

I thought green card holders weren’t allowed to travel outside the US.

[MeanWestTexan]

You need to present a passport from your country of citizenship to travel in/out of the country.

The issue is presumably he has no visa for the EU.

[MeanWestTexan]

And for clarity, you can leave and enter the US freely.

But the other country has to accept the passport of your country of citizenship.

[verum ago]

I thought green card holders weren’t allowed to travel outside the US.

From Customs and Immigration:

Permanent residents are free to travel outside the United States, and temporary or brief travel usually does not affect your permanent resident status. If it is determined, however, that you did not intend to make the United States your permanent home, you will be found to have abandoned your permanent resident status.

The usual rule of thumb is that they'll look at pulling a green card after a full calendar year outside the country (allows for things like taking care of a parent 'back home' toward the end of their life, etc, as long you still appear to be trying to maintain a domicile here.

Presumably he has a criminal issue here, or some issue on the EU end of the trip.

[Vlad0]

I thought green card holders weren’t allowed to travel outside the US.

I am not sure why you thought that, but it's not true.

One of the main advantages of having a Green Card is you can leave and return pretty much at will. You are not supposed to leave for more than six months at a time.

If you really need to you can get a travel permit if you need to stay outside the USA for longer, due to something like a family illness.

I have deep experience with this, I'm not guessing.