The attorneys who post answers here are divided about the response to your question. Many think the airlines check all outstanding warrants. In truth the airlines don't check anything: they give their flight manifest to Homeland Security who then checks for people with terrorist affiliations or people on the "no fly" list. Generally state criminal matters do not come to their attention. However in your case I'd be concerned about the size of the warrant and whether that might trigger any checks by Homeland Security.
You most likely won't have a problem leaving the country. HOWEVER, there is a good chance that you will be arrested on the warrant when you go through customs on your return. Also, the warrant may come up when you are processed through customs in Europe, as was the case with a client of mine who went to Canada. He was promptly put on a flight home and was arrested when he arrived.
Both TSA and ICE have access to a wide array of criminal investigation information, so there is certainly a chance you will be stopped, questioned and perhaps placed in custody at the airport. You might advise the person to check with a local lawyer to clear up the pending warrant before departure and to take immediate steps to upgrade status. With Federal authorities charging Umar Farouk Abdulmutallab with attempting to destroy Flight 253 on its final approach to Detroit Metropolitan Airport on Christmas Day, you know that airport security is raised to a very high alert leve domesticallyl.
You need a lawyer. Check with a lawyer in your locale to discuss more of the details.
You might find my Legal Guide helpful " What Do I Tell My Lawyer?"
Good luck to you.
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