Hello and thank you for using Just Answer.The legal part of surrendering Citizenship requires that your mother personally sit down with an Embassy or Consul representative to discuss the repercussions of surrendering her citizenship.
Generally, the actual act of expatriation for a U.S. citizen for tax purposes occurs when an individual formally renounces U.S. citizenship.
IRC § 877A subjects a covered expatriate to an income tax on the net unrealized gain on their worldwide property as if the property had been sold for fair market value on the day before expatriation. Losses may be taken into account, but only to the extent of gains. Thus, application of the expatriation rules may not result in a loss for tax purposes. The first $600,000 (indexed annually) of net gain is excluded for each expatriating individual.If your mother has been filing her returns and does not have more than the $600,000 then she will not owe more tax based on her decision.