First - I am not a Square representative but have some experience with non-citizens working in the US. The FEIN is your version of a U.S. social security number (SSN), and is the number your U.S. taxes are reported/paid under, so you would use this anytime something requires your 'SSN'. Should never use another persons SSN for any reason, and they should never agree to have someone else use their SSN, because anything tied to your transaction and their SSN comes back to them personally.
Hope this helps
