We had the same situation and we explained to her that she needed to deal with it or it could possibly hinder her goal of being a citizen

Just send a letter back to them telling them the situation and giving her email and/or physical address.

I always urge my au pairs to complete the tax formalities under the (mis?)Impression that it might help them one day to be able to prove they paid their taxes while here, ie if they they try to come back on another type of visa or marry an american etc. I'd try to get her to sign it, make a copy and mail it in - better safe than sorry.

Our Au Pair who left in March had her 2019 tax return sent back to our house today because she didn't sign the form. They seem to have cashed her check but returned the form for signature. Has this happened to anyone else after departure? I could forward it to her I guess but if they have her money already...isn't this just a formality?