Get it done "before" the expiration date specified in that letter. Remember, an amended return can not be e-filed. The IRS says so. You have to print, sign and mail it to the IRS.
Additionally, if you filed a state return, then removing her as a dependent from the federal return will mean you need to do the same for the state return.
Next, even if your mother has no income she will have to file a return to show the 1095-A she is covered on, to the IRS.
