Hi there,
Your son's wife filed her return incorrectly.
If they are married and living together, There are only two ways to file, Married filing Jointly (MFJ) or Married Filing Separately (MFS). You cannot file single if you are lawfully married.
She will have to amend her return to file either MFJ or MFS.

There are many advantages to filing a joint return. One is a larger standard deduction, and you potentially qualify for certain tax breaks that are not afforded to those who file MFS, but there are a few circumstances when MFS is best.

Either way, because they are married, neither could file single. She must amend and file a joint return (file together). If she wants to file Married Filing Separately, then your son must also file Married Filing Separately.