Retirement tax questions

I think your answer is what I have been looking for!  My sister is the qualifying person with the 401 K that she has not withdrawn from as it would be too early and she would incur a penalty.  It was how that 401 K would be treated when I used the filing status as HOH.  I believe she qualifies on all other tests, but I wasn't sure and couldn't find the narrow answer I needed.  What is your source of reference for your answer to me so I can read up and further understand this.  Thank you very much.