Hello @joeystarr and welcome to Connect Head and Neck Cancer group. I would think the reference to radiation breaking the jaw is most likely the possibility of osteoradionecrosis, which occurs in a limited number of patients and usually after several years. A lot depends on the location of the beam projection and the type of radiation beam used (proton being the most accurate).
Feel free to look through the many posts in this group, the search component can be a great help as it is geared to areas within this group primarily.
You are a good friend to help someone through this difficult and stressful life changing journey. Feel free to post questions or start your own Discussion, which alerts others to offer up help and advice.

Never heard of that happening. Did he have surgery first?
I had a neck dissection and lingual tonsils removed.
Jaw will be sore and there are jaw exercises. Had a speech therapist lined up as part of the recovery. Best of luck