Yes, undocumented worker.  That's what I was saying all along.  What you mentioned about treaty test allowing Mexican to work in the US is incorrect and irrelevant.  If you didn't withhold these key facts, I could have given you a clear answer much earlier.

Provided your client was living in the US, albeit illegally, during all of 2019, he/she is a resident alien by substantial presence test and will be subject to tax on a worldwide basis like any US citizen.  A F.1040 is what you need to file for your client.