Please consult with your attorney, since you already have one. If the attorney represents the employer, and you are concerned about them finding out, then seek advice in-person on this matter as opposed to soliciting an answer online. It would be good to know what the arrest was for as well. Also, you should not require an expungment if you were not convicted.
Lastly, your employer can do a lot of things if you are a at-will employee.
Legal disclaimer: (714) 479-1000 Mr. Zapata's response to your question is general in nature, as not all the facts are known to him. You should retain an attorney experienced in immigration law to review all the facts in your case in order to receive advice specific to your case. Mr. Zapata's statement above does not create an attorney/client relationship.
You should consult with an immigration lawyer, not your employer's lawyer, at this time. When you go to the appointment, take your arrest and acquittal records and all of your immigration records with you. See the link below to AILA for help in finding an immigration lawyer near you. It doesn't sound like you are under any obligation at this time to disclose the incident, but , when you file your I-485 Application for Adjustment of Status, you will be asked to disclose the arrest. Make sure you get certified copies of the police report and the court disposition, not guilty, record.
You need to disclose the arrest to your employer's attorney. There will be additional forms that need to be filed in the future, which will require you to indicate whether or not you were arrested. You will also need to submit a certificate of disposition. An expungement will not help for purposes of immigration as you must disclose the arrest. Your attorney cannot provide accurate advice and properly represent you without knowing all the facts. As for what might be the employment consequences, I can provide no input.
Wendy R. Barlow, Esq, The Law Offices of Grinberg & Segal, P.L.L.C., 111 Broadway, Suite 1306, New York NY 10006, (866) 456-8654, email@example.com, www.myattorneyusa.com. The information contained in this answer is provided for informational purposes only, and should not be construed as legal advice on any subject matter. No recipients of content from this answer, clients or otherwise, should act or refrain from acting on the basis of any content included in the answer without seeking the appropriate legal or other professional advice on the particular facts and circumstances at issue from a licensed attorney. Provision of information on this website does not create an attorney-client relationship between you and The Law Offices of Grinberg & Segal, P.L.L.C., nor is it intended to do so.
You should discuss this issue with your immigration attorney and take all relevant reports and documents so that he can discuss issues related to whether you are still remain admissible. As for your employer, it depends on whether you have an employment contract, whether that contract has a clause that addresses your brush with the law and .... These are two separate issues but if your employer decides that they no longer want to "sponsor you", you will certainly have worries related to your immigration case for that, if nothing else. Be honest and upfront with your attorney.