I was also HPV negative. The reason why they gave me the rad option was that the tumor was entirely removed with clear margins in the tongue and all of the nodes removed through the neck dissection came back negative for cancer.

My surgeon never anticipated that rads would not be necessary as my PET was positive on both sides of my neck for lymph node involvement (turns out a false positive).

At the end of the day, your mom will have to weigh the pros and cons of each option, but I feel very fortunate to not have had to go through radiation.

Good luck!