If the tax liability relates to a year that your mom filed jointly with her husband, then she is also liable for these taxes. Having said that, it seems odd based on the limited facts that you have provided that the letter was addressed only to him.
If she is held liable, she may be able to avail herself of the "innocent spouse" rules to avoid liability. These are complicated and fact sensitive rules and she will need a tax attorney to help her.
Hope this helps.
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